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

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

You can use ECMAScript 6's String.prototype.startsWith() method. It's supported in all major browsers. However, if you want to use it in a browser that is unsupported you'll want to use a shim/polyfill to add it on those browsers. 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

This looks backwards through haystack for an occurrence of needle starting from index string length of haystack back to zero. In other words, it only checks if haystack starts with needle. lastIndexOf provides a second optional parameter 'fromIndex'. If given, the backwards search starts at this given index position and traverses back to index zero. But we must not specify any other fromIndex than the very last index, otherwise the search might overlook something.

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.