javascript default function value

Question

Is there any reason why something like this would not work?

var classReplace = function(object, newClass, originalClass = "") {
    //do stuff
}

I keep getting a "Uncaught SyntaxError: Unexpected token = " error because I add in the

originalClass = ""

part

It won't work because JavaScript has no such language feature (at present anyway). — Pointy
– Pointy, Commented Apr 7, 2014 at 23:28
ES6 will have it: wiki.ecmascript.org/…. "Is there any reason why something like this would not work?" Because the language doesn't allow it. — Felix Kling
– Felix Kling, Commented Apr 7, 2014 at 23:33
so this is wrong? developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — Michael McLaughlin
– Michael McLaughlin, Commented Apr 8, 2014 at 0:02
Warning: For Gecko 15 and upper versions. developer.mozilla.org/en-US/docs/Mozilla/Gecko — Mosho
– Mosho, Commented Apr 8, 2014 at 0:09

Travis J · Accepted Answer · 2014-04-07 23:30:58Z

2

You can check to see if originalClass was defined and if not then assign it "",

var classReplace = function(object, newClass, originalClass) {
 if( typeof(originalClass) === "undefined" ) originalClass = "";
 //do stuff
}

answered Apr 7, 2014 at 23:30

Travis J

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Kylok · Accepted Answer · 2014-04-07 23:35:56Z

1

As Pointy said, setting a default argument value in this way isn't possible in JavaScript,

You can, however, achieve similar results by checking if said argument is undefined, and if so, setting it equal to your desired default value:

var classReplace = function(object, newClass, originalClass) {
    if (originalClass === undefined) originalClass = "";
    //do stuff
}

answered Apr 7, 2014 at 23:35

Kylok

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Comments

Mosho · Accepted Answer · 2014-04-07 23:40:52Z

While this doesn't exist in JS out of the box, I can think of two options in addition to the other answers, that actually mimick the feature you are looking for.

a. CoffeeScript

Among other cool features, CS supports default function values:

var classReplace = (object, newClass, originalClass = "") ->
console.log(originalClass)

Of course this just gets interpreted as:

var classReplace = function(object, newClass, originalClass) {
  if (originalClass == null) {
    originalClass = "";
  }
  return console.log(originalClass);
};

But still nice to have for readability, and like I said, CS has a lot of other cool features, may be worth a look.

b. Lo-Dash's partialRight method. (link)

usage:

var originalFunction = function(a,b) { return a + b; },
    functionWithDefaultValues = _.partialRight(originalFunction, 1, 2);

jsfiddle

notes:

_.partial, for some reason, doesn't have the same behaviour.
_.partialRight appends the values from the right, so in the above, the default value for b is 1.

Hope this helps.

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