Hướng dẫn javascript indexof array

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Syntax

indexOf(searchElement)
indexOf(searchElement, fromIndex)

Parameters

searchElement

Element to locate in the array.

The index to start the search at. If the index is greater than or equal to the array's length, -1 is returned, which means the array will not be searched. If the provided index value is a negative number, it is taken as the offset from the end of the array. Note: if the provided index is negative, the array is still searched from front to back. If the provided index is 0, then the whole array will be searched. Default: 0 (entire array is searched).

Return value

The first index of the element in the array; -1 if not found.

Description

indexOf() compares searchElement to elements of the Array using strict equality (the same method used by the === or triple-equals operator).

The indexOf() method skips empty slots in sparse arrays.

Examples

Using indexOf()

The following example uses indexOf() to locate values in an array.

const array = [2, 9, 9];
array.indexOf(2);     // 0
array.indexOf(7);     // -1
array.indexOf(9, 2);  // 2
array.indexOf(2, -1); // -1
array.indexOf(2, -3); // 0

Finding all the occurrences of an element

const indices = [];
const array = ['a', 'b', 'a', 'c', 'a', 'd'];
const element = 'a';
let idx = array.indexOf(element);
while (idx !== -1) {
  indices.push(idx);
  idx = array.indexOf(element, idx + 1);
}
console.log(indices);
// [0, 2, 4]

Finding if an element exists in the array or not and updating the array

function updateVegetablesCollection (veggies, veggie) {
  if (veggies.indexOf(veggie) === -1) {
    veggies.push(veggie);
    console.log(`New veggies collection is: ${veggies}`);
  } else {
    console.log(`${veggie} already exists in the veggies collection.`);
  }
}

const veggies = ['potato', 'tomato', 'chillies', 'green-pepper'];

updateVegetablesCollection(veggies, 'spinach');
// New veggies collection is: potato,tomato,chillies,green-pepper,spinach
updateVegetablesCollection(veggies, 'spinach');
// spinach already exists in the veggies collection.

Using indexOf() on sparse arrays

You cannot use indexOf() to search for empty slots in sparse arrays.

console.log([1, , 3].indexOf(undefined)); // -1

See also