String.prototype.startsWith() - JavaScript 编辑

The startsWith() method determines whether a string begins with the characters of a specified string, returning true or false as appropriate.

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Syntax

str.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 to 0.

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

Browser compatibility

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See also

• String.prototype.endsWith()
• String.prototype.includes()
• String.prototype.indexOf()
• String.prototype.lastIndexOf()