[javascript] JavaScript variable number of arguments to function

Is there a way to allow "unlimited" vars for a function in JavaScript?

Example:

load(var1, var2, var3, var4, var5, etc...)
load(var1)

In (most) recent browsers, you can accept variable number of arguments with this syntax:

function my_log(...args) {
     // args is an Array
     console.log(args);
     // You can pass this array as parameters to another function
     console.log(...args);
}

Here's a small example:

function foo(x, ...args) {
  console.log(x, args, ...args, arguments);
}

foo('a', 'b', 'c', z='d')

=>

a
Array(3) [ "b", "c", "d" ]
b c d
Arguments
?    0: "a"
    ?1: "b"
    ?2: "c"
    ?3: "d"
    ?length: 4

Documentation and more examples here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters

It is preferable to use rest parameter syntax as Ramast pointed out.

function (a, b, ...args) {}

I just want to add some nice property of the ...args argument

1. It is an array, and not an object like arguments. This allows you to apply functions like map or sort directly.
2. It does not include all parameters but only the one passed from it on. E.g. function (a, b, ...args) in this case args contains argument 3 to arguments.length

Although I generally agree that the named arguments approach is useful and flexible (unless you care about the order, in which case arguments is easiest), I do have concerns about the cost of the mbeasley approach (using defaults and extends). This is an extreme amount of cost to take for pulling default values. First, the defaults are defined inside the function, so they are repopulated on every call. Second, you can easily read out the named values and set the defaults at the same time using ||. There is no need to create and merge yet another new object to get this information.

function load(context) {
   var parameter1 = context.parameter1 || defaultValue1,
       parameter2 = context.parameter2 || defaultValue2;

   // do stuff
}

This is roughly the same amount of code (maybe slightly more), but should be a fraction of the runtime cost.

Use the arguments object when inside the function to have access to all arguments passed in.

While @roufamatic did show use of the arguments keyword and @Ken showed a great example of an object for usage I feel neither truly addressed what is going on in this instance and may confuse future readers or instill a bad practice as not explicitly stating a function/method is intended to take a variable amount of arguments/parameters.

function varyArg () {
    return arguments[0] + arguments[1];
}

When another developer is looking through your code is it very easy to assume this function does not take parameters. Especially if that developer is not privy to the arguments keyword. Because of this it is a good idea to follow a style guideline and be consistent. I will be using Google's for all examples.

Let's explicitly state the same function has variable parameters:

function varyArg (var_args) {
    return arguments[0] + arguments[1];
}

Object parameter VS var_args

There may be times when an object is needed as it is the only approved and considered best practice method of an data map. Associative arrays are frowned upon and discouraged.

SIDENOTE: The arguments keyword actually returns back an object using numbers as the key. The prototypal inheritance is also the object family. See end of answer for proper array usage in JS

In this case we can explicitly state this also. Note: this naming convention is not provided by Google but is an example of explicit declaration of a param's type. This is important if you are looking to create a more strict typed pattern in your code.

function varyArg (args_obj) {
    return args_obj.name+" "+args_obj.weight;
}
varyArg({name: "Brian", weight: 150});

Which one to choose?

This depends on your function's and program's needs. If for instance you are simply looking to return a value base on an iterative process across all arguments passed then most certainly stick with the arguments keyword. If you need definition to your arguments and mapping of the data then the object method is the way to go. Let's look at two examples and then we're done!

Arguments usage

function sumOfAll (var_args) {
    return arguments.reduce(function(a, b) {
        return a + b;
    }, 0);
}
sumOfAll(1,2,3); // returns 6

Object usage

function myObjArgs(args_obj) {
    // MAKE SURE ARGUMENT IS AN OBJECT OR ELSE RETURN
    if (typeof args_obj !== "object") {
        return "Arguments passed must be in object form!";
    }

    return "Hello "+args_obj.name+" I see you're "+args_obj.age+" years old.";
}
myObjArgs({name: "Brian", age: 31}); // returns 'Hello Brian I see you're 31 years old

Accessing an array instead of an object ("...args" The rest parameter)

As mentioned up top of the answer the arguments keyword actually returns an object. Because of this any method you want to use for an array will have to be called. An example of this:

Array.prototype.map.call(arguments, function (val, idx, arr) {});

To avoid this use the rest parameter:

function varyArgArr (...var_args) {
    return var_args.sort();
}
varyArgArr(5,1,3); // returns 1, 3, 5

Yes, just like this :

function load()
{
  var var0 = arguments[0];
  var var1 = arguments[1];
}

load(1,2);

Another option is to pass in your arguments in a context object.

function load(context)
{
    // do whatever with context.name, context.address, etc
}

and use it like this

load({name:'Ken',address:'secret',unused:true})

This has the advantage that you can add as many named arguments as you want, and the function can use them (or not) as it sees fit.

Be aware that passing an Object with named properties as Ken suggested adds the cost of allocating and releasing the temporary object to every call. Passing normal arguments by value or reference will generally be the most efficient. For many applications though the performance is not critical but for some it can be.

As mentioned already, you can use the arguments object to retrieve a variable number of function parameters.

If you want to call another function with the same arguments, use apply. You can even add or remove arguments by converting arguments to an array. For example, this function inserts some text before logging to console:

log() {
    let args = Array.prototype.slice.call(arguments);
    args = ['MyObjectName', this.id_].concat(args);
    console.log.apply(console, args);
}

I agree with Ken's answer as being the most dynamic and I like to take it a step further. If it's a function that you call multiple times with different arguments - I use Ken's design but then add default values:

function load(context) {

    var defaults = {
        parameter1: defaultValue1,
        parameter2: defaultValue2,
        ...
    };

    var context = extend(defaults, context);

    // do stuff
}

This way, if you have many parameters but don't necessarily need to set them with each call to the function, you can simply specify the non-defaults. For the extend method, you can use jQuery's extend method ($.extend()), craft your own or use the following:

function extend() {
    for (var i = 1; i < arguments.length; i++)
        for (var key in arguments[i])
            if (arguments[i].hasOwnProperty(key))
                arguments[0][key] = arguments[i][key];
    return arguments[0];
}

This will merge the context object with the defaults and fill in any undefined values in your object with the defaults.