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

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

• 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