To get the index of an element in an array in JavaScript - ECMAScript 2021, we can use the indexOf() and findIndex() methods. Here are some examples:
1. Using indexOf() method:
The indexOf() method returns the first index at which a given element can be found in the array, or -1 if it is not present.
const fruits = ["apple", "banana", "orange", "mango"];
// Get index of "banana"
console.log(fruits.indexOf("banana")); // Output: 1
// Get index of "pear", which is not present in array
console.log(fruits.indexOf("pear")); // Output: -1
2. Using findIndex() method:
The findIndex() method returns the index of the first element in the array that satisfies the provided testing function. Otherwise, it returns -1.
const people = [
{ name: "John", age: 25 },
{ name: "Mary", age: 30 },
{ name: "Peter", age: 35 },
{ name: "Ann", age: 20 }
];
// Find index of Peter
const index = people.findIndex(person => person.name === "Peter");
console.log(index); // Output: 2
// Find index of person whose age is greater than 30
const ageIndex = people.findIndex(person => person.age > 30);
console.log(ageIndex); // Output: 2
Note: Both indexOf() and findIndex() methods return the index of the first occurrence of the element that matches the specified criteria. However, indexOf() only works with primitive types (strings, numbers, etc.), while findIndex() can work with any type of array elements.