Javascript add extra argument

Question

Lets take a look at this code:

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

The arguments that are passed to \"mainF

Answer 1

One liner to add additional argument(s) and return the new array:

[].slice.call(arguments).concat(['new value']));

Answer 2

The arguments "array" isn't an array (it's a design bug in JavaScript, according to Crockford), so you can't do that. You can turn it into an array, though:

var mainFunction = function() {
  var mainArguments = Array.prototype.slice.call(arguments);
  mainArguments.push('extra data');

  altFunction.apply(null, mainArguments);
}

Answer 3

For those who like me was looking for a way to add an argument that is optional and may be not the only omitted one (myFunc = function(reqiured,optional_1,optional_2,targetOptional) with the call like myFunc(justThisOne)), this can be done as follows:

// first we make sure arguments is long enough
// argumentPosition is supposed to be 1,2,3... (4 in the example above)
while(arguments.length < argumentPosition)
    [].push.call(arguments,undefined);
// next we assign it
arguments[argumentPosition-1] = arguments[argumentPosition-1] || defaultValue;

Answer 4

var mainFunction = function() {
    var args = [].slice.call( arguments ); //Convert to array
    args.push( "extra data");
    return altFunction.apply( this, args );
}

Answer 5

Update 2016: You must convert the arguments to an array before adding the element. In addition to the slice method mentioned in many posts:

var args = Array.prototype.slice.call(arguments);

You can also use the Array.from() method or the spread operator to convert arguments to a real Array:

var args = Array.from(arguments);

or

var args = [...arguments];

The above may not be optimized by your javascript engine, it has been suggested by the MDN the following may be optimized:

var args = (arguments.length === 1 ? [arguments[0]] : Array.apply(null, arguments));

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments