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


Question

How would I write the equivalent of C#'s String.StartsWith in JavaScript?

var haystack = 'hello world';
var needle = 'he';

haystack.startsWith(needle) == true

Note: This is an old question, and as pointed out in the comments ECMAScript 2015 (ES6) introduced the .startsWith method. However, at the time of writing this update (2015) browser support is far from complete.

2018/09/08
1
1702
9/8/2018 8:54:31 PM

Accepted Answer

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:

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:

"Hello World!".startsWith("He"); // true

var haystack = "Hello world";
var prefix = 'orl';
haystack.startsWith(prefix); // false
1780
5/7/2019 3:56:14 AM


data.substring(0, input.length) === input
2011/06/06

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.

2017/05/23

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
2020/03/27

I just wanted to add my opinion about this.

I think we can just use like this:

var haystack = 'hello world';
var needle = 'he';

if (haystack.indexOf(needle) == 0) {
  // Code if string starts with this substring
}
2016/02/12

Here is a minor improvement to CMS's solution:

if(!String.prototype.startsWith){
    String.prototype.startsWith = function (str) {
        return !this.indexOf(str);
    }
}

"Hello World!".startsWith("He"); // true

 var data = "Hello world";
 var input = 'He';
 data.startsWith(input); // true

Checking whether the function already exists in case a future browser implements it in native code or if it is implemented by another library. For example, the Prototype Library implements this function already.

Using ! is slightly faster and more concise than === 0 though not as readable.

2015/11/18

Source: https://stackoverflow.com/questions/646628
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