15.5.4.6 String.prototype.concat([string1[, string2[, ... ]]])

string String.prototype.concat([string1:string|mixed[, string2:string|mixed[, ... ]]])

Combines this and all the arguments, all coerced to string, to a new string (primitive).

String.prototype.concat.length = 1

String.prototype.concat = function(){
CheckObjectCoercible(this);
var S = ToString(this);
var args = Array.prototype.slice.call(arguments);
var R = S;
while (args.length) {
var next = args.shift();
var R = R + ToString(next);
}
}