Default argument values in JavaScript functions [duplicate]

Question

In javascript you can call a function (even if it has parameters) without parameters.

So you can add default values like this:

function func(a, b){
   if (typeof(a)==='undefined') a = 10;
   if (typeof(b)==='undefined') b = 20;

   //your code
}

and then you can call it like func(); to use default parameters.

Here's a test:

function func(a, b){
   if (typeof(a)==='undefined') a = 10;
   if (typeof(b)==='undefined') b = 20;

   alert("A: "+a+"\nB: "+b);
}
//testing
func();
func(80);
func(100,200);

ES2015 onwards:

From ES6/ES2015, we have default parameters in the language specification. So we can just do something simple like,

function A(a, b = 4, c = 5) {
}

or combined with ES2015 destructuring,

function B({c} = {c: 2}, [d, e] = [3, 4]) {
}

For detailed explanation,

https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/default_parameters

Default function parameters allow formal parameters to be initialized with default values if no value or undefined is passed.

Pre ES2015:

If you're going to handle values which are NOT Numbers, Strings, Boolean, NaN, or null you can simply use

(So, for Objects, Arrays and Functions that you plan never to send null, you can use)

param || DEFAULT_VALUE

for example,

function X(a) {
  a = a || function() {};
}

Though this looks simple and kinda works, this is restrictive and can be an anti-pattern because || operates on all falsy values ("", null, NaN, false, 0) too - which makes this method impossible to assign a param the falsy value passed as the argument.

So, in order to handle only undefined values explicitly, the preferred approach would be,

function C(a, b) {
  a = typeof a === 'undefined' ? DEFAULT_VALUE_A : a;
  b = typeof b === 'undefined' ? DEFAULT_VALUE_B : b;
}

You have to check if the argument is undefined:

function func(a, b) {
    if (a === undefined) a = "default value";
    if (b === undefined) b = "default value";
}

Also note that this question has been answered before.

I have never seen it done that way in JavaScript. If you want a function with optional parameters that get assigned default values if the parameters are omitted, here's a way to do it:

 function(a, b) {
      if (typeof a == "undefined") {
        a = 10;
      }

      if (typeof b == "undefined") {
        a = 20;
      }

      alert("a: " + a + " b: " + b);
    }

function func(a, b)
{
  if (typeof a == 'undefined')
    a = 10;
  if (typeof b == 'undefined')
    b = 20;
  // do what you want ... for example
  alert(a + ',' + b);
}

in shorthand

function func(a, b)
{
  a = (typeof a == 'undefined')?10:a;
  b = (typeof b == 'undefined')?20:b;

  // do what you want ... for example
  alert(a + ',' + b);
}