What does the substr() method do in JavaScript?

The substr() method extracts a portion of a string, starting from a specified index position, and returns a new string with a specified number of characters.

Does substr() modify the original string?

No, substr() does not modify the original string. It returns a new string containing the extracted characters, leaving the original string unchanged.

Can you use negative numbers with substr()?

Yes, you can use negative numbers with substr(). A negative start index indicates the position from the end of the string. For example, str.substr(-3, 2) extracts 2 characters starting from the 3rd character from the end.

How does substr() handle out-of-bounds indices?

If the start index provided to substr() is greater than or equal to the string's length, it returns an empty string. If the length parameter makes the extraction extend beyond the string's length, it extracts characters until the end of the string.

Are there scenarios where substr() might be less efficient?

substr() may be less efficient when used repeatedly in loops or when extracting very large portions of a string. In such cases, using techniques like caching results or using more efficient string manipulation methods might improve overall performance.

Are there scenarios where substr() might be less efficient?

We have a Cheat Sheet on JavaScript where we covered all the important topics of JavaScript to check those please go through JavaScript Cheat Sheet – A Basic Guide to JavaScript.

JavaScript String substring() Method

JavaScript String substr() Method

Last Updated : 22 Mar, 2025