Make includes() case insensitive in JavaScript

# Table of Contents

1. Make includes() case insensitive in JavaScript
2. Ignoring case Check if Array contains String - Array.findIndex
3. Ignoring Case Check if Array contains String - Array.some()
4. Ignoring Case Check if Array contains String - Array.find()
5. Perform a case-insensitive check if a string is in an array using filter()

# Make includes() case insensitive in JavaScript

To make the String.includes() method case insensitive, convert both of the strings in the comparison to lowercase.

A case-insensitive comparison is done by converting the two strings to the same case.

Copied!
const str = 'HELLO WORLD';
const substr = 'hELLo';

// 👇️ true
console.log(str.toLowerCase().includes(substr.toLowerCase()));

if (str.toLowerCase().includes(substr.toLowerCase())) {
  // 👇️ this runs
  console.log('The substring is contained in the string');
} else {
  console.log('The substring is NOT contained in the string');
}

We converted both strings to lowercase to perform a case-insensitive lookup using the String.includes method.

We could achieve the same result by converting the strings to uppercase.

Copied!
const str = 'HELLO WORLD';
const substr = 'hELLo';

// 👇️ true
console.log(str.toUpperCase().includes(substr.toUpperCase()));

if (str.toUpperCase().includes(substr.toUpperCase())) {
  // 👇️ this runs
  console.log('The substring is contained in the string');
} else {
  console.log('The substring is NOT contained in the string');
}

We just need the strings to be in the same case to be able to determine if the substring is contained in the string.

# Ignoring case Check if Array contains String - Array.findIndex

To do a case insensitive check if an array contains a string:

1. Use the Array.findIndex() method to iterate over the array.
2. Convert each array element and the string to lowercase and check for equality.
3. If the condition is met, the element's index is returned.

Copied!
const names = ['John', 'Bob', 'Adam'];
const name = 'ADAM';

const index = names.findIndex(element => {
  return element.toLowerCase() === name.toLowerCase();
});

console.log(index); // 👉️ 2

if (index !== -1) {
  // 👇️ this runs
  console.log('The string is contained in the array');
} else {
  console.log('The string is NOT contained in the array');
}

The function we passed to the Array.findIndex method gets called with each element in the array until it returns a truthy value or iterates over the entire array.

Once the callback function returns a truthy value, the element's index is returned from findIndex().

The function in the example returned 2 because the 3rd element in the array matched the string.

If the function we passed to findIndex never returns a truthy value, then findIndex returns -1.

Copied!
const names = ['John', 'Bob', 'Adam'];
const name = 'Tim';

const result = names.findIndex(element => {
  return element.toLowerCase() === name.toLowerCase();
});
console.log(result); // 👉️ -1

Alternatively, you can use the Array.some() method.

# Ignoring Case Check if Array contains String - Array.some()

This is a three-step process:

1. Use the Array.some() method to iterate over the array.
2. Convert each array element and the string to lowercase and check for equality.
3. If the condition is met, true is returned.

Copied!
const names = ['John', 'Bob', 'Adam'];
const name = 'JOHN';

const includesValue = names.some(element => {
  return element.toLowerCase() === name.toLowerCase();
});

console.log(includesValue); // 👉️ true

The difference between Array.findIndex() and Array.some() is that some returns true or false, instead of the index of the array element or -1.

The function we passed to the Array.some() method is called with each element of the array until a truthy value is returned or the array's values are exhausted.

An alternative would be to convert both strings to uppercase to achieve the same result.

Alternatively, you can use the Array.find() method.

# Ignoring Case Check if Array contains String - Array.find()

This is a three-step process:

1. Use the Array.find() method to iterate over the array.
2. Convert each array element and the string to lowercase and check for equality.
3. If the condition is met, the matching element is returned.

Copied!
const names = ['John', 'Bob', 'Adam'];
const name = 'ADAM';

const result = names.find(element => {
  return element.toLowerCase() === name.toLowerCase();
});

console.log(result); // 👉️ 'Adam'

The function we passed to the Array.find method gets called with each element in the array until it returns a truthy value or iterates over the entire array.

If the callback function returns a truthy value, the corresponding element is returned from the Array.find() method.

If all invocations of the callback function return a falsy value, the Array.find() method returns undefined.

# Perform a case-insensitive check if a string is in an array using filter()

If you need to perform a case-insensitive check whether a string is contained in an array and get all matches:

1. Use the Array.filter() method to iterate over the array.
2. On each iteration, lowercase the array element and the string it should be compared to.
3. The Array.filter method will return an array of all of the elements that satisfy the condition

Copied!
const arr = ['HELLO', 'HeLlO', 'WORLD'];
const str = 'HeLLo';
const matches = arr.filter(element => {
  return element.toLowerCase() === str.toLowerCase();
});

console.log(matches); // 👉️ ['HELLO', 'HeLlO']

if (matches.length > 0) {
  // 👉️ at least 1 match found in the array
}

The function we passed to the Array.filter method gets called with each element of the array.

The Array.filter() method does not stop iterating once the callback function returns a truthy value. It returns an array of all elements that satisfy the condition.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Replace a String case-insensitive in JavaScript
• Sort an Array of strings ignoring the Case in JavaScript
• TypeError: Cannot use 'in' operator to search for 'X' in 'Y'