Why is lodash.each faster than native forEach?

Question

I was trying to find the fastest way of running a for loop with its own scope. The three methods I compared were:

var a = \"t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t         


        

Answer 1

Yes, lodash/underscore each don't even have same semantics as .forEach. There is a subtle detail that will make the function really slow unless the engine can check for sparse arrays without getters quickly.

This will be 99% spec compliant and runs at the same speed as lodash each in V8 for the common case:

function FastAlmostSpecForEach( fn, ctx ) {
    "use strict";
    if( arguments.length > 1 ) return slowCaseForEach();
    if( typeof this !== "object" ) return slowCaseForEach();
    if( this === null ) throw new Error("this is null or not defined");
    if( typeof fn !== "function" ) throw new Error("is not a function");
    var len = this.length;
    if( ( len >>> 0 ) !== len ) return slowCaseForEach();


    for( var i = 0; i < len; ++i ) {
        var item = this[i];
        //Semantics are not exactly the same,
        //Fully spec compliant will not invoke getters
       //but this will.. however that is an insane edge case
        if( item === void 0 && !(i in this) ) {
            continue;
        }
        fn( item, i, this );
    }
}

Array.prototype.fastSpecForEach = FastAlmostSpecForEach;

By checking for undefined first, we don't punish normal arrays in the loop at all. An engine could use its internals to detect strange arrays but V8 doesn't.