Javascript add extra argument

Question

Let's take a look at this code:

var mainFunction = function() {   altFunction.apply(null, arguments); } 

The arguments that are passed to mainFunction are dynamic -- they can be 4 or 10, doesn't matter. However, I have to pass them through to altFunction AND I have to add an EXTRA argument to the argument list.

I have tried this:

var mainFunction = function() {   var mainArguments = arguments;   mainArguments[mainArguments.length] = 'extra data'; // not +1 since length returns "human" count.    altFunction.apply(null, mainArguments); } 

But that does not seem to work. How can I do this?

Answer 1

Use Array.prototype.push

[].push.call(arguments, "new value"); 

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There's no need to shallow clone the arguments object because it and its .length are mutable.

(function() {     console.log(arguments[arguments.length - 1]); // foo      [].push.call(arguments, "bar");      console.log(arguments[arguments.length - 1]); // bar })("foo"); 

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From ECMAScript 5, 10.6 Arguments Object

7. Call the [[DefineOwnProperty]] internal method on obj passing "length", the Property Descriptor {[[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}, and false as arguments.

So you can see that .length is writeable, so it will update with Array methods.

Answer 2

arguments is not a pure array. You need to make a normal array out of it:

var mainArguments = Array.prototype.slice.call(arguments); mainArguments.push("extra data");