functional programming - Is it possible to rewrite JavaScript's apply function? -

i've been rewriting lot of javascript's higher-order functions hang of functional programming, , i'm stuck on apply. possible write apply in javascript? assuming other native functions present, , es5 spec being used.

with es5 , below, don't think can without using eval (see below). can almost massive switch statment on args.length, @ point, have there's limit number of cases in switch.

function.prototype.newapply = function(thisarg, args) {     switch (args.length) {         case 0: return this.call(thisarg);         case 1: return this.call(thisarg, args[0]);         case 2: return this.call(thisarg, args[0], args[1]);         // etc..         default: throw new error("she canna tek more!");     } }; 

if you're allowing eval, though, can absolutely it — full credit blex suggesting eval:

function.prototype.newapply = function(thisarg, args) {     var f = this,         call = "f.call(thisarg",         i;     (i = 1; < args.length; ++i) {         call += ", args[" + + "]";     }     call += ")";     return eval(call); }; 

live example:

function.prototype.newapply = function(thisarg, args) {    var f = this,        call = "f.call(thisarg",        i;    (i = 0; < args.length; ++i) {      call += ", args[" + + "]";    }    call += ")";    return eval(call);  };    var obj1 = {    foo: "foo",    bar: "bar"  };  var obj2 = {    foo: "f",    bar: "b"  };  function caps(o1, o2) {    var k;    snippet.log("this.x = " + this.x);    (k in o1) {      o1[k] = o1[k].touppercase();    }    (k in o2) {      o2[k] = o2[k].tolowercase();    }  }    caps.newapply({x:42}, [obj1, obj2]);  snippet.log(json.stringify(obj1));  snippet.log(json.stringify(obj2));

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or if want use array#reduce:

function.prototype.newapply = function(thisarg, args) {     var f = this,         call = args.reduce(function(acc, _, index) {             return acc + ", args[" + index + "]";         }, "f.call(thisarg") + ")";     return eval(call); }; 

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you said es5 in question, completeness: it's easy in es6 spread operator (...):

function.prototype.newapply = function(thisarg, args) {     return this.call(thisarg, ...args); };