Javascript startsWith prototype

Question

I'm a Java guy in a JavaScript world, and I'm used to certain things about Java Strings that just aren't there for in JavaScript. Some of this I can accept, for example Strings are freaking everywhere for some reason.

Now I realize this could be simply implemented with return this.indexOf(needle) === 0; (or, even more JavaScripty, return this.indexOf(needle); but the flaw in that is that it will search the entire (potentially somewhat lengthy, you never know) String in its endeavor to find the index, when I really only care if its non-zero.

My current solution, which passes my testing so far, is thus. I'm open to all feedback, but I'm most interested in

• Breaks from conventional JavaScript (the bad stuff, the smells)
• Corner-cases this fails for
• To a reasonable extent, style

I've also decided to tack it onto the class itself (I have to admit, I like being able to do that!) Is that bad practice?

String.prototype.startsWith = function(needle) {
    // This was dubious, if only because most times "" gets passed
    // it's by mistake... but decided since it was technically correct
    // that it should stay
    if(needle === "") return true;
    // but in all other cases, falsey values were considered false
    if(!needle) return false;

    // our mission, to find the needle in the haystack - is this tacky?
    var haystack = this;

    // primarily here to prevent AIOOBEs in the loop - is this the most
    // JavaScripty way to preempt that error?
    if(needle.length > haystack.length) {
        return false;
    }
    // I've read that 'i' is not scoped to this for loop, and that I would
    // need let instead. I must support older browsers, is there a better
    // way to do this?
    for(var i = 0; i < needle.length; i++) {
        if(haystack.charAt(i) !== needle.charAt(i)) {
            return false;
        }
    }
    return true;

}

Answer 1

I'd suggest using String.prototype.slice to implement startsWith like this:

String.prototype.startsWith = function(str) {
  return this.slice(0, str.length) === str
}

For type-checking (not needed in this example, but generally speaking) you might want to use the typeof operator:

if (typeof str !== 'string') return false

You can also use the instanceof operator if the value your dealing w/ isn't a literal or you need to check every value in the prototype chain, e.g. str instanceof String

haystack = this? Super-tacky ;p, but each to their own

Not sure if JS has an AIOOBE equivalent but 'blah'.charAt(6) === '' which tells me that check isn't needed.

And I'd suggest using Array iterators rather than for loops in the future:

var haystackStartsWithNeedle = needle.split('').
  forEach((v,i) => v === haystack[i]).find(v => v)

You might want to check babeljs.io. It transpiles new & flavoured JS to code most browsers can use.