Default value for function parameter?

Question

So I've been doing this for as long as I can remember, but I'm curious if this is really what I should be doing. You write a function that takes a parameter, so you anticipate it to have a value, but if it doesn't, you have a good reason to default it, to say zero. What I currently do is write a helper function:

function foo() { return foo(0); };
function foo(bar) { ... };

I just ran across an instance where I did this and I looked at it oddly for a few seconds before understanding my logic behind it. I come from php where it's trivial:

function foo(bar=0) { ... }

Is there a javascript alternative that I'm not aware of?

Answer 1

You can't have overloaded functions in JavaScript. Instead, use object based initialization, or check for the values and assign a default if none supplied.

In your example, the second function foo(bar) will replace the first one.

Here's a function using object initialization.

function foo(config) {
    extend(this, config);
}

where extend is a function that merges the config object with the current object. It is similar to the $.extend method in jQuery, or $extend method of MooTools.

Invoke the function and pass it named key value pairs

foo({ bar: 0 });

The other way to initialize is to look at the supplied values, and assign a default if the value is not given

function foo(bar) {
    bar = bar || 0;
}

This works as long as bar is not a falsy value. So foo(false) or foo("") will still initialize bar to 0 . For such cases, do an explicit check.

function foo(bar) {
    bar = (typeof bar == 'undefined' ? 0 : bar);
}

Answer 2

In JavaScript, the argument will be undefined if the user didn't pass it in. You can use the || operator to set the value of the argument if it's undefined:

function foo(bar) {
  bar = bar || 0;
  ...
}

Answer 3

The simplest way I know of is test for a value and then set it to a default value if no value is found. I have not come across a catch all one liner yet, this is the best i have got.

If expecting a string value use this. Default will trigger on these values: [ undefined, null, "" ]

function foo(str) {
  str = !!str  ? str  : 'bar';
  ...
}

If expecting a number or Boolean value. This allows 0 and false as values. Default will trigger on [ undefined, null, {}, functions ]

Handy for making values arguments that only accept primitive values like number, boolean and string

function foo(val) {
  val= !!val == val || val*1 ? val : 10;
  ...
}

If you're looking to test for objects such as {}, There is documentation on doing this but it isn't so simple.

Answer 4

Hopefully this answers a bit clearer for someone - I ended up using the ol' check for undefined if(typeof functionparameter !== 'undefined') as per:

$.fn.extend({
    doThing: function(stringparameter = 'FooBar!', functionparameter){
        console.log('Here is your string '+stringparameter);
        // Run it if it's been passed through:
        if(typeof functionparameter !== 'undefined') functionparameter();
});