How to check if a string "StartsWith" another string?

Question

You can use ECMAScript 6's String.prototype.startsWith() method, but it's not yet supported in all browsers. You'll want to use a shim/polyfill to add it on browsers that don't support it. Creating an implementation that complies with all the details laid out in the spec is a little complicated. If you want a faithful shim, use either:

• Matthias Bynens's String.prototype.startsWith shim, or
• The es6-shim, which shims as much of the ES6 spec as possible, including String.prototype.startsWith.

Once you've shimmed the method (or if you're only supporting browsers and JavaScript engines that already have it), you can use it like this:

   console.log("Hello World!".startsWith("He")); // true
    
var haystack = "Hello world";
var prefix = 'orl';
console.log(haystack.startsWith(prefix)); // false

Another alternative with .lastIndexOf:

haystack.lastIndexOf(needle, 0) === 0

This looks backwards through haystack for an occurrence of needle starting from index 0 of haystack. In other words, it only checks if haystack starts with needle.

In principle, this should have performance advantages over some other approaches:

• It doesn't search the entire haystack.
• It doesn't create a new temporary string and then immediately discard it.

data.substring(0, input.length) === input

Without a helper function, just using regex's .test method:

/^He/.test('Hello world')

To do this with a dynamic string rather than a hardcoded one (assuming that the string will not contain any regexp control characters):

new RegExp('^' + needle).test(haystack)

You should check out Is there a RegExp.escape function in Javascript? if the possibility exists that regexp control characters appear in the string.

Best solution:

function startsWith(str, word) {
    return str.lastIndexOf(word, 0) === 0;
}

And here is endsWith if you need that too:

function endsWith(str, word) {
    return str.indexOf(word, str.length - word.length) !== -1;
}

For those that prefer to prototype it into String:

String.prototype.startsWith || (String.prototype.startsWith = function(word) {
    return this.lastIndexOf(word, 0) === 0;
});

String.prototype.endsWith   || (String.prototype.endsWith = function(word) {
    return this.indexOf(word, this.length - word.length) !== -1;
});

Usage:

"abc".startsWith("ab")
true
"c".ensdWith("c") 
true

With method:

startsWith("aaa", "a")
true
startsWith("aaa", "ab")
false
startsWith("abc", "abc")
true
startsWith("abc", "c")
false
startsWith("abc", "a")
true
startsWith("abc", "ba")
false
startsWith("abc", "ab")
true