Optional argument first in JavaScript

Optional argument first in JavaScript

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For the user's ease, I have a function that receives an optional argument first, before the required argument. For example:

ns.myFunction('optional arg', function(){
    //the required callback

I'm doing this rather than doing the following since the callback body could be long, and the user might forget to override the defaults to the optional arguments:

    //the required callback
}, 'optional arg');

Currently I'm doing this to check:

function myFunction(first, second) {

    //if second is undefined and first is a function
    if (typeof second === 'undefined' && typeof first === 'function') { 
        second = first;


  • Is this the right way?
  • How would I do it so that it scales, especially if there were N optional arguments that are before the required argument?

This is not the right way because optional parameters are by convention always placed at the end. And you see a reason why: it is much easier to handle them. If the length of anonymous function is your concern, clients of your API should use function references or variables:

function callback1() { //...

var callback2 = function() {//...

myFunction(callbackX, optional);

The problem with escaping this can be solved with bind().

If you really want to go the path of multiple optional parameters and callback at the end, I can think of two ways: arguments object or wrapping all optional arguments in one options objects.

With arguments you can write:

var lastOptionalIndex = arguments.length - 2;
var callback = arguments[lastOptionalIndex + 1];  //required callback is always last
var optionalFirst = lastOptionalIndex >=0? arguments[0] : undefined;
var optionalSecond = lastOptionalIndex >=1? arguments[1] : undefined;

See how ugly it is compared to:

function myFunction(callback, firstOptional, secondOptional //...

With options wrapper object you always have two arguments:

function myFunction(options, callback);

Where options is just an object:

  firstOptional: 1,
  secondOptional: 'foo'

Default parameters, The optional argument is limit where you specify how many times you want to split the string. So, if I only wanted to split the string to the first instance of the  The optional argument is limit where you specify how many times you want to split the string. So, if I only wanted to split the string to the first instance of the separator, I would just provide a 1.

It can easilly be done with ArgueJS:

function myFunction (){
  arguments = __({first: [String], second: Function})

  // and now on, you can access your arguments by
  //   arguments.first and arguments.second

A very similar example can be found at ArgueJS' first example. Note that only the parameter in the middle is required:

function range(){ 
  arguments = __({start: [Number, 0], stop: Number, step: [Number, 1]})

  for(var i = arguments.start; i < arguments.stop; i += arguments.step)


>>> range(3)
>>> range(3, 5)
>>> range(0, 5, 2)

How JavaScript Optional Arguments Work, Optional parameters are a nice language feature - function parameters that are given At first glance, javascript has nothing like this available. This method only allows the last arguments to be optional - you cannot make an optional first parameter, middle parameter, or combination of parameters optional. The next methods let you position optional arguments anywhere. Method 2: The arguments variable. All javascript functions get passed an implicit arguments variable when they're called.

It's not necessary to name any of the parameters. you can simply say:

if (arguments.length < 2)  // There are no optional parameters

And retrieve each parameter via arguments[i]. The function is found at arguments[arguments.length - 1].

On a minor note, the typeof operator always returns a string, so == can be used instead of ===

Optional Parameters in Javascript, In the first function call, we didn't pass any argument into the say() function, therefore message parameter took the default value 'Hi' . In the second function call,  Optional Function Arguments in JavaScript Many programming languages support optional function arguments - ie. functions in which certain arguments are optional. You can set a default value for any argument - so if the argument is not present, the default value will be used.


function range(){ 
  arguments = __({start: [Number, 0], stop: Number, step: [Number, 1]});
  for(var i = arguments.start; i < arguments.stop; i += arguments.step)

Become even prettier with ES6:

function range(){ 
  {start, stop, step} = __({start: [Number, 0], stop: Number, step: [Number, 1]})
  for(var i = start; i < stop; i += step)

Or CoffeeScript:

range = ->
  {start, stop, step} = __ {start: [Number, 0], stop: Number, step: [Number, 1]}
  console.log i for i in [start...stop] by step

The Beginner's Guide to JavaScript Default Parameters, At first, this may just seem like some syntactic sugar on assignment statements, but what makes it much more than that is the ability to assign default values to the​  If you can destructure function parameters, and you can assign default values during that destructuring, this means that you can simulate named and optional arguments in JavaScript.

You can configurate function arguments, depending on their number:

myFunction() {
    let first, second;
    if (arguments.length === 2) {
        [first, second] = arguments;
    } else if (arguments.length === 1) {
        [second] = arguments;
    if ( ! first) {
        // first is undefined, so only second argument was passed.

ES6 destructuring makes code clean and suitable for scaling.

Named and Optional Arguments in JavaScript - DailyJS, length; ++i) { args[i] = arguments[i]; }; // first argument is the error object // shift() removes the first item from the // array and returns it err = args.shift  A function implementation having a default param can be assigned to a function type having corresponding param optional. For example: For example: let f: (x: number, y?: number) => void; f = function (x: number, y: number = 2): void {//y is default console.log(`x= ${x}, y=${y}`); } f(3);

How to use optional arguments in functions (with optional , JavaScript optional parameters allows Java developers to declare optional one param, so function will consider it the first value of parameter. In order for JavaScript functions to be able to do the same thing with different input, they need a way for programmers to give them input. The difference between parameters and arguments can be confusing at first. Here’s how it works: Parameters are the names you specify in the function definition. Arguments are the values …

Optional Parameters In JavaScript, In this approach, the optional parameter is Logically ORed with the default value within the body of the function. Note: The optional parameters should always  console.log(typeof arguments); // 'object' The type of individual arguments can be determined by indexing arguments: console.log(typeof arguments[0]); // returns the type of the first argument Properties arguments.callee Reference to the currently executing function that the arguments belong to. arguments.length

How to declare the optional function parameters in JavaScript , The current JavaScript specification doesn't support function parameter default Where as in some other programming languages, parameter default values might be If the parameter might also be false, check the type first with typeof : Arguments are Passed by Value. The parameters, in a function call, are the function's arguments. JavaScript arguments are passed by value: The function only gets to know the values, not the argument's locations. If a function changes an argument's value, it does not change the parameter's original value.