Javascript Array indexOf()

Introduction

The indexOf() method finds the first index for a given element.

It returns -1 if not found. 

arr.indexOf(searchElement[, fromIndex])
  • searchElement - element to locate in the array.
  • fromIndex - Optional, The index to start. Default to 0.

If fromIndex is greater than or equal to the array's length, -1 is returned.

Search an array for the item "Java": 

var languages = ["CSS", "HTML", "Java", "Javascript"];
var a = languages.indexOf("Java");
console.log(a);//from  ww  w .  j a v a  2s  .c o  m

Search an array for the item "Java", starting the search at position 4: 

var languages = ["CSS", "HTML", "Java", "Javascript", "CSS", "HTML", "Java", "Javascript"];
var a = languages.indexOf("Java", 4);
console.log(a);//from w  w w  . j a  v a2  s.com

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

var array = [2, 6, 9, 9];
let a = array.indexOf(2);     
console.log(a);/* w w  w .ja  v  a 2 s.  c  o m*/
a = array.indexOf(7);     
console.log(a);
a = array.indexOf(6, 2);  
console.log(a);
a = array.indexOf(2, -1); 
console.log(a);
a = array.indexOf(2, -3); 
console.log(a);

Finding all the occurrences of an element 

var indices = [];
var array = ['a', 'b', 'a', 'c', 'a', 'd', 'e','a'];
var element = 'a';
var idx = array.indexOf(element);
while (idx != -1) {
  indices.push(idx);//from  w  w  w  .  j  a v  a2 s  .  c  om
  idx = array.indexOf(element, idx + 1);
}
console.log(indices);

More example 

let numbers = [1, 2, 3, 4, 5, 4, 3, 2, 1];

console.log(numbers.indexOf(4)); // 3 
console.log(numbers.lastIndexOf(4)); // 5 
console.log(numbers.includes(4)); // true 

console.log(numbers.indexOf(4, 4)); // 5 
console.log(numbers.lastIndexOf(4, 4)); // 3 
console.log(numbers.includes(4, 7)); // false 

let person = {//from ww  w .  j a  v  a2  s.  com
    name: "HTML"
};
let people = [{
    name: "HTML"
}];
let morePeople = [person];

console.log(people.indexOf(person)); // -1 
console.log(morePeople.indexOf(person)); // 0 
console.log(people.includes(person)); // false 
console.log(morePeople.includes(person)); // true



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