Can you declare facultative function arguments in Javascript ?

Question

Like you can in php

<?php
function whatever($var='') {

}
?>

can you do that in javascript?

Answer 1

In javascript you can call a function regardless of parameters.

In other words it's perfectly legal to declare a function like this:

 function func(par1, par3) {

   //do something
  }

and call it like this:

 func();

Answer 2

If you want to set a 'default' value to your arguments, you can do something like this:

function whatever(arg1) {
  arg1 = arg1 || 'default value';
}

Keep in mind that the 'default value' will be set if arg1 contains any falsy value, like null, undefined, 0, false, NaN or a zero-length string "".

Also in JavaScript functions you have the arguments object, it's an array-like object that contains the arguments passed to a function, therefore you can even declare a function without input arguments, and when you call it, you can pass some to it:

function whatever() {
  var arg1 = arguments[0];
}

whatever('foo');

Edit: For setting a default value only if it is truly undefined, as @bobbymcr comments, you could do something like this also:

function whatever(arg1) {
  arg1 = arg1 === undefined ? 'default value' : arg1;
}

Answer 3

In JS by default arguments aren't required. For $arg = '' implementation you can use

function whatever(vars) {  
   vars = vars || 'default';  
   /* your code */    
}

Answer 4

I don't think you can do this directly, but there are ways to achieve this. Since JavaScript lets you omit parameters from a function call, you can check if a parameter is undefined and give it a default value if so:

function DoSomething(x, y, z)
{
    // set default values...

    if (typeof x == "undefined")
    {
        x = 5;
    }

    if (typeof y == "undefined")
    {
        y = "Hi";
    }

    if (typeof z == "undefined")
    {
        z = 3.14;
    }

    // ...
}

You can try calling it in the following ways to see the default values get assigned when the parameter is missing:

DoSomething();
DoSomething(4);
DoSomething(4, "X");
DoSomething(4, "X", 7.77);