Make Array.indexOf() case insensitive in JavaScript

How to make Array.indexOf() case insensitive #

To get the index of an element by performing case insensitive lookup, we have to:

1. call the Array.findIndex method with a function
2. the function should convert the array element and the string to lowercase and do an equality check
3. the Array.findIndex method returns the element's index or -1 if no elements satisfy the condition

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// Not Supported in IE 6-11
const arr = ['HELLO', 'WORLD'];
const str = 'world';

const index = arr.findIndex(element => {
  return element.toLowerCase() === str.toLowerCase();
});

console.log(index); // 👉️ 1

if (index !== -1) {
  // 👉️ string is in the array
}

The function that we passed to the Array.findIndex method gets invoked with each element of the array until it returns a truthy value or iterates over all elements.

The string is contained in the array at index 1, therefore that's the return value from Array.findIndex.

If the equality comparison never returns true, then Array.findIndex will return -1.

Note that Array.findIndex is not supported in Internet Explorer 6-11, so if you have to support the browser, make sure to get a polyfill or use babel to transpile your code to an older version of JavaScript that the browser understands.

Further Reading #

• Check if Array of Strings contains a Substring in JS
• Replace a String case insensitive in JavaScript
• Find all Uppercase characters in a String using JavaScript
• Check if First Letter of String is Uppercase in JavaScript
• Ignore case of startsWith, endsWith Methods in JavaScript