How to check if a string “StartsWith” another stri

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.

Answer 1

You can also return all members of an array that start with a string by creating your own prototype / extension to the the array prototype, aka

Array.prototype.mySearch = function (target) {
    if (typeof String.prototype.startsWith != 'function') {
        String.prototype.startsWith = function (str){
        return this.slice(0, str.length) == str;
      };
    }
    var retValues = [];
    for (var i = 0; i < this.length; i++) {
        if (this[i].startsWith(target)) { retValues.push(this[i]); }
    }
    return retValues;
};

And to use it:

var myArray = ['Hello', 'Helium', 'Hideout', 'Hamster'];
var myResult = myArray.mySearch('Hel');
// result -> Hello, Helium

Answer 2

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
}

Answer 3

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

Answer 4

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.