Get key, value map javascript

The get() method returns a specified element from a Map object. If the value that is associated to the provided key is an object, then you will get a reference to that object and any change made to that object will effectively modify it inside the Map object.

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Syntax

Parameters

key

The key of the element to return from the Map object.

Return value

The element associated with the specified key, or undefined if the key can't be found in the Map object.

Examples

Using get()

const myMap = new Map();
myMap.set('bar', 'foo');

console.log(myMap.get('bar')); // Returns "foo"
console.log(myMap.get('baz')); // Returns undefined

Using get() to retrieve a reference to an object

const arr = [];
const myMap = new Map();
myMap.set('bar', arr);

myMap.get('bar').push('foo');

console.log(arr); // ["foo"]
console.log(myMap.get('bar')); // ["foo"]

Note that the map holding a reference to the original object effectively means the object cannot be garbage-collected, which may lead to unexpected memory issues. If you want the object stored in the map to have the same lifespan as the original one, consider using a WeakMap.

Specifications

Specification
ECMAScript Language Specification
# sec-map.prototype.get

Browser compatibility

BCD tables only load in the browser

See also

The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and primitive values) may be used as either a key or a value.

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Description

Map objects are collections of key-value pairs. A key in the Map may only occur once; it is unique in the Map's collection. A Map object is iterated by key-value pairs — a for...of loop returns a 2-member array of [key, value] for each iteration. Iteration happens in insertion order, which corresponds to the order in which each key-value pair was first inserted into the map by the set() method (that is, there wasn't a key with the same value already in the map when set() was called).

The specification requires maps to be implemented "that, on average, provide access times that are sublinear on the number of elements in the collection". Therefore, it could be represented internally as a hash table (with O(1) lookup), a search tree (with O(log(N)) lookup), or any other data structure, as long as the complexity is better than O(N).

Key equality

Value equality is based on the SameValueZero algorithm. (It used to use SameValue, which treated 0 and -0 as different. Check browser compatibility.) This means NaN is considered the same as NaN (even though NaN !== NaN) and all other values are considered equal according to the semantics of the === operator.

Objects vs. Maps

Object is similar to Map—both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. For this reason (and because there were no built-in alternatives), Object has been used as Map historically.

However, there are important differences that make Map preferable in some cases:

Setting object properties

Setting Object properties works for Map objects as well, and can cause considerable confusion.

Therefore, this appears to work in a way:

const wrongMap = new Map();
wrongMap['bla'] = 'blaa';
wrongMap['bla2'] = 'blaaa2';

console.log(wrongMap); // Map { bla: 'blaa', bla2: 'blaaa2' }

But that way of setting a property does not interact with the Map data structure. It uses the feature of the generic object. The value of 'bla' is not stored in the Map for queries. Other operations on the data fail:

wrongMap.has('bla')    // false
wrongMap.delete('bla') // false
console.log(wrongMap)  // Map { bla: 'blaa', bla2: 'blaaa2' }

The correct usage for storing data in the Map is through the set(key, value) method.

const contacts = new Map()
contacts.set('Jessie', {phone: "213-555-1234", address: "123 N 1st Ave"})
contacts.has('Jessie') // true
contacts.get('Hilary') // undefined
contacts.set('Hilary', {phone: "617-555-4321", address: "321 S 2nd St"})
contacts.get('Jessie') // {phone: "213-555-1234", address: "123 N 1st Ave"}
contacts.delete('Raymond') // false
contacts.delete('Jessie') // true
console.log(contacts.size) // 1

Constructor

Map()

Creates a new Map object.

Static properties

get Map[@@species]

The constructor function that is used to create derived objects.

Instance properties

Map.prototype.size

Returns the number of key/value pairs in the Map object.

Instance methods

Map.prototype.clear()

Removes all key-value pairs from the Map object.

Map.prototype.delete()

Returns true if an element in the Map object existed and has been removed, or false if the element does not exist. map.has(key) will return false afterwards.

Map.prototype.get()

Returns the value associated to the passed key, or undefined if there is none.

Map.prototype.has()

Returns a boolean indicating whether a value has been associated with the passed key in the Map object or not.

Map.prototype.set()

Sets the value for the passed key in the Map object. Returns the Map object.

Map.prototype[@@iterator]()

Returns a new Iterator object that contains a two-member array of [key, value] for each element in the Map object in insertion order.

Map.prototype.keys()

Returns a new Iterator object that contains the keys for each element in the Map object in insertion order.

Map.prototype.values()

Returns a new Iterator object that contains the values for each element in the Map object in insertion order.

Map.prototype.entries()

Returns a new Iterator object that contains a two-member array of [key, value] for each element in the Map object in insertion order.

Map.prototype.forEach()

Calls callbackFn once for each key-value pair present in the Map object, in insertion order. If a thisArg parameter is provided to forEach, it will be used as the this value for each callback.

Examples

Using the Map object

const myMap = new Map();

const keyString = 'a string';
const keyObj = {};
const keyFunc = function() {};

// setting the values
myMap.set(keyString, "value associated with 'a string'");
myMap.set(keyObj, 'value associated with keyObj');
myMap.set(keyFunc, 'value associated with keyFunc');

console.log(myMap.size); // 3

// getting the values
console.log(myMap.get(keyString)); // "value associated with 'a string'"
console.log(myMap.get(keyObj)); // "value associated with keyObj"
console.log(myMap.get(keyFunc)); // "value associated with keyFunc"

console.log(myMap.get('a string')); // "value associated with 'a string'", because keyString === 'a string'
console.log(myMap.get({})); // undefined, because keyObj !== {}
console.log(myMap.get(function() {})); // undefined, because keyFunc !== function () {}

Using NaN as Map keys

NaN can also be used as a key. Even though every NaN is not equal to itself (NaN !== NaN is true), the following example works because NaNs are indistinguishable from each other:

const myMap = new Map();
myMap.set(NaN, 'not a number');

myMap.get(NaN);
// "not a number"

const otherNaN = Number('foo');
myMap.get(otherNaN);
// "not a number"

Iterating Map with for...of

Maps can be iterated using a for...of loop:

const myMap = new Map();
myMap.set(0, 'zero');
myMap.set(1, 'one');

for (const [key, value] of myMap) {
  console.log(`${key} = ${value}`);
}
// 0 = zero
// 1 = one

for (const key of myMap.keys()) {
  console.log(key);
}
// 0
// 1

for (const value of myMap.values()) {
  console.log(value);
}
// zero
// one

for (const [key, value] of myMap.entries()) {
  console.log(`${key} = ${value}`);
}
// 0 = zero
// 1 = one

Iterating Map with forEach()

Maps can be iterated using the forEach() method:

myMap.forEach((value, key) => {
  console.log(`${key} = ${value}`);
});
// 0 = zero
// 1 = one

Relation with Array objects

const kvArray = [['key1', 'value1'], ['key2', 'value2']];

// Use the regular Map constructor to transform a 2D key-value Array into a map
const myMap = new Map(kvArray);

console.log(myMap.get('key1')); // "value1"

// Use Array.from() to transform a map into a 2D key-value Array
console.log(Array.from(myMap)); // Will show you exactly the same Array as kvArray

// A succinct way to do the same, using the spread syntax
console.log([...myMap]);

// Or use the keys() or values() iterators, and convert them to an array
console.log(Array.from(myMap.keys())); // ["key1", "key2"]

Cloning and merging Maps

Just like Arrays, Maps can be cloned:

const original = new Map([
  [1, 'one'],
]);

const clone = new Map(original);

console.log(clone.get(1)); // one
console.log(original === clone); // false (useful for shallow comparison)

Note: Keep in mind that the data itself is not cloned.

Maps can be merged, maintaining key uniqueness:

const first = new Map([
  [1, 'one'],
  [2, 'two'],
  [3, 'three'],
]);

const second = new Map([
  [1, 'uno'],
  [2, 'dos'],
]);

// Merge two maps. The last repeated key wins.
// Spread syntax essentially converts a Map to an Array
const merged = new Map([...first, ...second]);

console.log(merged.get(1)); // uno
console.log(merged.get(2)); // dos
console.log(merged.get(3)); // three

Maps can be merged with Arrays, too:

const first = new Map([
  [1, 'one'],
  [2, 'two'],
  [3, 'three'],
]);

const second = new Map([
  [1, 'uno'],
  [2, 'dos'],
]);

// Merge maps with an array. The last repeated key wins.
const merged = new Map([...first, ...second, [1, 'eins']]);

console.log(merged.get(1)); // eins
console.log(merged.get(2)); // dos
console.log(merged.get(3)); // three

Specifications

Specification
ECMAScript Language Specification
# sec-map-objects

Browser compatibility

BCD tables only load in the browser

See also

  • A polyfill of Map is available in core-js
  • Set
  • WeakMap
  • WeakSet

How can I get a value in a JavaScript map by its key?

To get value for a specific key in Map in JavaScript, call get() method on this Map and pass the specific key as argument. get() method returns the corresponding value for given key, if present. If the specified key is not present, then get() returns undefined.

How do I find a key

import java. util. HashMap; import java. util. Map;.
public static K getKey(Map map, V value) {.
for (K key: map. keySet()) {.
if (value. equals(map. get(key))) { return key;.
return null; }.
public static void main(String[] args) {.
Map hashMap = new HashMap(); hashMap. put("A", 1);.

How do you get a Keys map in TypeScript?

We use the get() method of a map in TypeScript to get the value for a key in a map. A map is a key-value pair data structure.

How do you pass a key

of("key1", "a", "key2", "b", "key3", "c"); Stream the entry set of map2. Use that entry's key as the key to the result map. use that entry's value as the key to retrieve the value of map1.