Why doesn't indexOf work on an array IE8?

javascript

polyfills

ie8-compatibility

javascript-features

byAnton Shumikhin·Mar 13, 2025

In IE8, indexOf is missing for arrays as it is a newer ECMAScript 5 feature. The following polyfill ensures that it functions:

if (!Array.prototype.indexOf) {
  Array.prototype.indexOf = function(searchElement) {
    for (var i = 0; i < this.length; i++) {
      if (this[i] === searchElement) return i; // Found it, victory dance!
    }
    return -1; // We couldn’t find it, better luck next time!
  };
}

Add the snippet in your code base to make indexOf work in IE8.

Putting it to work: Using polyfill in IE8

As IE8 falls short in ECMAScript 5 features, including .indexOf(), polyfills or alternative methods are necessary for this functionality.

Exploring substitutes: jQuery's $.inArray()

If you're leveraging jQuery, the function $.inArray() is an effective alternative:

var index = $.inArray(searchElement, array); // jQuery stepping in to save the day!

Impressively, this function provides an IE8-compatible alternative to indexOf.

Comparison: $.inArray() and indexOf

Although $.inArray() comes to the rescue, it's crucial to understand its purpose and limits:

It deals with arrays, not strings.
Unlike indexOf, $.inArray() returns -1 for non-existent elements, or the item's index starting from 0.

Understanding such nuances prevents hiccups in your JavaScript journey.

Best practices for IE8 compatibility

To handle the limitations of legacy browsers like IE8, implement these techniques:

Feature detection: Check if a function exists before putting it to use.
Conditional comments: Access specific IE versions for scripts or styles.
Shims and polyfills: Develop or include scripts that add missing functionality.

With these methods in your toolbelt, you'll be coding flexibly and efficient, no matter the browser.

Essential references for JavaScript developers

As developers, we must stay updated and informed. Two key resources for JavaScript are:

MDN documentation for a deep understanding of JavaScript functions and browser compatibility.
"Can I use..." to check if a modern web feature is supported in older browsers like IE8.

Knowledge is power, especially when you anticipate and solve issues ahead of time.

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