String.startsWith()
Checks if a string starts with a substring (true/false).
string.startsWith(searchString, position?)This static page keeps the syntax and examples indexed for search, while the coding app handles interactive exploration and saved references.
What it does
Overview
Checks if a string starts with a substring (true/false).
The startsWith() method determines whether a string begins with the characters of a specified search string, returning true or false as appropriate. This method is case-sensitive and does not support regular expressions as the search pattern. Performance-wise, it is highly efficient for prefix validation compared to methods like indexOf() == 0 or substring comparisons, as it stops execution as soon as a mismatch is found. Edge cases include passing an empty string as the search pattern, which always returns true, or providing a negative position, which is treated as 0. If the position parameter is greater than or equal to the length of the string, the method returns false (unless the search string is empty). This method was introduced in ES6, so Internet Explorer is not supported.
Quick reference
Syntax
string.startsWith(searchString, position?)Inputs
Parameters
See it in practice
Examples
Basic Prefix Validation
const protocol = 'https://api.example.com';
console.log(protocol.startsWith('https'));true
Determines if a URL string begins with the secure protocol prefix.
Searching from a Custom Position
const phrase = 'The quick brown fox';
console.log(phrase.startsWith('quick', 4));true
Using the optional position parameter to check if a word exists at a specific index.
Case-Insensitive Prefix Check
const filename = 'README.md';
const search = 'readme';
const result = filename.toLowerCase().startsWith(search.toLowerCase());
console.log(result);true
Normalization using toLowerCase() is required because startsWith() is natively case-sensitive.
Debug faster
Common Errors
TypeError
Cause: Passing a Regular Expression as the searchString argument.
Fix: Use a plain string for startsWith(), or use RegExp.test() with the '^' anchor if regex functionality is required.
'hello'.startsWith(/he/);TypeError
Cause: Calling startsWith on a null or undefined reference.
Fix: Ensure the variable is a string or use optional chaining or a null check before calling the method.
var str = null;
str.startsWith('a');Runtime support
Compatibility
Source: MDN Web Docs
Common questions
Frequently Asked Questions
Checks if a string starts with a substring (true/false).
searchString: Prefix to check for. position: Index to start checking from (default 0).
TypeError: Use a plain string for startsWith(), or use RegExp.test() with the '^' anchor if regex functionality is required. TypeError: Ensure the variable is a string or use optional chaining or a null check before calling the method.