String.prototype.startsWith()
The startsWith()
method
determines whether a string begins with the characters of a specified string,
returning true
or false
as appropriate.
Syntax
startsWith(searchString)
startsWith(searchString, position)
Parameters
searchString
-
The characters to be searched for at the start of this string.
position
Optional-
The position in this string at which to begin searching for
searchString
. Defaults to0
.
Return value
true
if the given characters are found at the beginning
of the string; otherwise, false
.
Description
This method lets you determine whether or not a string begins with another string. This method is case-sensitive.
Examples
Using startsWith()
//startswith
let str = 'To be, or not to be, that is the question.'
console.log(str.startsWith('To be')) // true
console.log(str.startsWith('not to be')) // false
console.log(str.startsWith('not to be', 10)) // true
Polyfill
This method has been added to the ECMAScript 2015 specification and may not be
available in all JavaScript implementations yet. However, you can polyfill
String.prototype.startsWith()
with the following snippet:
if (!String.prototype.startsWith) {
Object.defineProperty(String.prototype, 'startsWith', {
value: function(search, rawPos) {
var pos = rawPos > 0 ? rawPos|0 : 0;
return this.substring(pos, pos + search.length) === search;
}
});
}
A more robust (fully ES2015 specification compliant), but less performant and compact, Polyfill is available on GitHub by Mathias Bynens.
Specifications
Specification |
---|
ECMAScript Language Specification # sec-string.prototype.startswith |
Browser compatibility
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