Kiểm tra xem giá trị có đúng không JavaScript

ví dụ

sử dụng cơ bản

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

loại null

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

Trong lần triển khai JavaScript đầu tiên, các giá trị JavaScript được biểu diễn dưới dạng thẻ loại và giá trị. Thẻ loại cho các đối tượng là

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

Một bản sửa lỗi đã được đề xuất cho ECMAScript [thông qua tùy chọn tham gia], nhưng đã bị từ chối. Nó sẽ dẫn đến

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

Sử dụng toán tử mới

Tất cả các hàm xây dựng được gọi với

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

Cần dấu ngoặc đơn trong cú pháp

Toán tử

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

Tương tác với các biến chưa được khai báo và chưa được khởi tạo

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number["1"] === "number"; // Number tries to parse things into numbers
typeof Number["shoe"] === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String[1] === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean[1] === "boolean"; // Boolean[] will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! [logical NOT] operator are equivalent to Boolean[]

// Symbols
typeof Symbol[] === "symbol";
typeof Symbol["foo"] === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date[] === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean[true] === "object";
typeof new Number[1] === "object";
typeof new String["abc"] === "object";

// Functions
typeof function [] {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";