indexOf method

The indexOf method searches for an element in an array. In the first parameter, specify the element text. The method returns the number of the first found element, or -1 if there is no such element. The second optional parameter of the method specifies the position from which to start searching.

Syntax

javascriptarray.indexOf(element, [where to start]);

Example

Let's find the position of the first three in the array:

javascriptlet arr = [1, 2, 3, 3, 3, 4, 5];
let res = arr.indexOf(3);

console.log(res);

The code execution result:

javascript2

Example

Let now the element being checked is not in the array:

javascriptlet arr = [1, 2, 3, 4, 5];
let res = arr.indexOf(6);

console.log(res);

The code execution result:

javascript-1

Example

Let's start the search from a given position:

javascriptlet arr = [1, 2, 3, 4, 5, 3];
let res = arr.indexOf(3, 4);

console.log(res);

The code execution result:

javascript5

Frequent mistake

Sometimes the method is used to check if an element is in an array. In this case, it is easy to make a mistake if you check as follows:

javascriptif (arr.indexOf(3)) {
  console.log(true);
} else {
  console.log(false);
}

Such a check will work for all positions, except for zero - after all, it is interpreted as false.

Properly check for the presence of an element as follows:

javascriptif (arr.indexOf(3) !== -1) {
  console.log(true);
} else {
  console.log(false);
}

See also