StringprototypestartsWith

String.prototype.startsWith is a JavaScript method that determines whether a string begins with the characters of a specified string, returning true or false. It is a standard part of the String prototype introduced in ECMAScript 2015 (ES6) and is widely supported in modern environments, including browsers and Node.js.

The method takes two parameters. The first, searchString, is the string to look for at the start of the target string. The second, position, is optional and specifies the index in the target string at which to begin searching; if omitted, it defaults to 0. If position is negative, it is treated as 0. If position is greater than the length of the string, startsWith returns false. The comparison is done using the ToString conversion on searchString, so non-string values are converted to strings (for example, startsWith(123) checks for the substring "123").

startsWith is a case-sensitive prefix check and does not interpret searchString as a regular expression. It is distinct from methods like includes or indexOf, which search for a substring anywhere or at a given position respectively; startsWith only checks the prefix from the specified position.

Examples:

- "Hello world".startsWith("Hello") -> true

- "Hello world".startsWith("hello") -> false

- "abcdef".startsWith("def", 3) -> true

- "abcdef".startsWith("", 2) -> true

Performance and compatibility considerations: in environments without native support, a polyfill can implement equivalent behavior, typically using a slice or substring comparison. Developers often use startsWith for input validation, feature detection, and string parsing where a precise prefix match is required.