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)          


        

Answer 1

The best performant solution is to stop using library calls and just recognize that you're working with two arrays. A hand-rolled implementation is both short and also faster than every other solution I've seen here.

function startsWith2(str, prefix) {
    if (str.length < prefix.length)
        return false;
    for (var i = prefix.length - 1; (i >= 0) && (str[i] === prefix[i]); --i)
        continue;
    return i < 0;
}

For performance comparisons (success and failure), see http://jsperf.com/startswith2/4. (Make sure you check for later versions that may have trumped mine.)