Array.prototype.forEach alternative implementation parameters
-
27-10-2019 - |
Question
When working on my latest web application and needing to use the Array.forEach
function, I constantly found the following code used to add support to older browsers that do not have the function built in.
/**
* Copyright (c) Mozilla Foundation http://www.mozilla.org/
* This code is available under the terms of the MIT License
*/
if (!Array.prototype.forEach) {
Array.prototype.forEach = function(fun /*, thisp*/) {
var len = this.length >>> 0;
if (typeof fun != "function") {
throw new TypeError();
}
var thisp = arguments[1];
for (var i = 0; i < len; i++) {
if (i in this) {
fun.call(thisp, this[i], i, this);
}
}
};
}
I fully understand what the code does and how it works, but I always see it copied with the formal thisp
parameter commented out and having it be set as a local variable using arguments[1]
instead.
I was wondering if anyone knew why this change was made, because from what I can tell, the code would have worked fine with thisp
as a formal parameter rather than a variable?
Solution
Array.prototype.forEach.length
is defined as 1
, so implementation functions would be more native-like if they had their .length
property set to 1
too.
http://es5.github.com/#x15.4.4.18
The length property of the forEach method is 1.
(func.length
is the amount of argument that func
takes based on its definition.)
For func.length
to be 1
, you have to define func
to only take 1 argument. In the function itself you can always get all arguments with arguments
. However, by defining the function to take 1 argument, the .length
property is 1
. Therefore, it is more correct according to the specification.
OTHER TIPS
This will iterate through each of the values in the array without iterating over the string equivalent of prototype functions.
Array.prototype.forEach = function(fun /*, thisp*/) {
if (typeof fun != "function") {
throw new TypeError();
}
for(i = 0; i < this.length; i++){
...
}
}