indexOf

Return the first index at which a given element can be found.

Usage

var indexOf = require( '@stdlib/utils/index-of' );

indexOf( arr, searchElement[, fromIndex] )

Returns the first index at which a given element can be found.

var arr = [ 4, 3, 2, 1 ];

var idx = indexOf( arr, 3 );
// returns 1

If a searchElement is not present in an input array, the function returns -1.

var arr = [ 4, 3, 2, 1 ];

var idx = indexOf( arr, 5 );
// returns -1

By default, the implementation searches an input array starting from the first element. To start searching from a different element, specify a fromIndex.

var arr = [ 1, 2, 3, 4, 5, 2, 6 ];

var idx = indexOf( arr, 2, 3 );
// returns 5

If a fromIndex exceeds the input array length, the function returns -1.

var arr = [ 1, 2, 3, 4, 2, 5 ];

var idx = indexOf( arr, 2, 10 );
// returns -1

If a fromIndex is less than 0, the starting index is determined relative to the last index (with the last index being equivalent to fromIndex = -1).

var arr = [ 1, 2, 3, 4, 5, 2, 6, 2 ];

var idx = indexOf( arr, 2, -4 );
// returns 5

idx = indexOf( arr, 2, -1 );
// returns 7

If fromIndex is less than 0 and its absolute value exceeds the input array length, the function searches the entire input array.

var arr = [ 1, 2, 3, 4, 5, 2, 6 ];

var idx = indexOf( arr, 2, -10 );
// returns 1

The first argument is not limited to arrays, but may be any array-like object.

var str = 'bebop';

var idx = indexOf( str, 'o' );
// returns 3

Notes

  • Search is performed using strict equality comparison. Thus,

    var arr = [ 1, [ 1, 2, 3 ], 3 ];
    
    var idx = indexOf( arr, [ 1, 2, 3 ] );
    // returns -1
    
  • This implementation is not ECMAScript Standard compliant. Notably, the standard specifies that an array be searched by calling hasOwnProperty (thus, for most cases, incurring a performance penalty), and the standard does not accommodate a searchElement equal to NaN. In this implementation, the following is possible:

    // Locate the first element which is NaN...
    var arr = [ 1, NaN, 2, NaN ];
    
    var idx = indexOf( arr, NaN );
    // returns 1
    
    // Prototype properties may be searched as well...
    function Obj() {
        this[ 0 ] = 'beep';
        this[ 1 ] = 'boop';
        this[ 2 ] = 'woot';
        this[ 3 ] = 'bap';
        this.length = 4;
        return this;
    }
    Obj.prototype[ 2 ] = 'bop';
    
    var obj = new Obj();
    
    idx = indexOf( obj, 'bop' );
    // returns -1
    
    delete obj[ 2 ];
    
    idx = indexOf( obj, 'bop' );
    // returns 2
    

Examples

var indexOf = require( '@stdlib/utils/index-of' );

var arr;
var obj;
var str;
var idx;
var i;

// Arrays...
arr = new Array( 10 );
for ( i = 0; i < arr.length; i++ ) {
    arr[ i ] = i * 10;
}
idx = indexOf( arr, 40 );

console.log( idx );
// => 4


// Array-like objects...
obj = {
    '0': 'beep',
    '1': 'boop',
    '2': 'bap',
    '3': 'bop',
    'length': 4
};

idx = indexOf( obj, 'bap' );

console.log( idx );
// => 2


// Strings...
str = 'beepboopbop';

idx = indexOf( str, 'o' );

console.log( idx );
// => 5