substring vs substr in JavaScript

Similar to how the difference between the slice vs splice array methods can be hard to remember, it can also be hard to remember the difference between the substring and substr JavaScript string methods. Here’s a quick reference to help out with that.

TL;DR: substring takes a starting index and an end index while substr takes a starting index and a length of characters.

String.prototype.substring

The substring() method, all spelled out, returns a new string with a subset of the string. With one argument passed-in, we get the string starting from the specified index (inclusive) until the end of the string:

const myStr = 'Alligator';

const myNewStr = myStr.substring(2);

console.log(myNewStr); // ligator

With two arguments passed-in, we get a subset of the string from the starting index to the end index (exclusive):

const myStr = 'Alligator';

const myNewStr = myStr.substring(0, 3);

console.log(myNewStr); // All

String.prototype.substr

The substr() method is very similar, but the second argument is not for the end index, it’s for the amount of characters.

Here we want a 3-character string from a starting index of 2:

const myStr = 'Alligator';

const myNewStr = myStr.substr(2, 3);

console.log(myNewStr); // lig

Negative start index

Additionally, the first argument to substr can be a negative integer, in which case the start of the returned string is counted from the end of the string that the method is used on:

const myStr = 'Alligator';

const myNewStr = myStr.substr(-2);

console.log(myNewStr); // or

Same Result When Only One Argument

When only the first argument is used and is a positive integer, both substring and substr return the same value:

const myStr = 'Alligator';

const myNewStrViaSubstring = myStr.substring(3);

const myNewStrViaSubstr = myStr.substr(3);

console.log(myNewStrViaSubstring); // igator

console.log(myNewStrViaSubstr); // igator