Default Parameters in Function Expressions

Question

Default parameters in javscript function declerations can be achieved with a simple assignment as follows:

function foo(arg1 = 'default1', arg2 = 'default2') { ... }

But how can I have default parameters for function expressions, as the following does NOT work in chrome v47:

var foo = function (arg1 = 'default1', arg2 = 'default2') { ... }

Any pointers would be helpful.

Your second piece of code works. What makes you think it doesn't? — Pointy, Feb 08 '16 at 15:15
Huh!? Then why am I getting a "Uncaught SyntaxError: Unexpected token =" error in the browser console? — Grateful, Feb 08 '16 at 15:16
Which browser are you using? Default values for function arguments are an ES2015 thing. — Pointy, Feb 08 '16 at 15:17
Chrome has not implemented this yet. Try it in a Firefox console. — mikhail, Feb 08 '16 at 15:26
I am NOT going to change my browser to have this work. I would much rather change the syntax! — Grateful, Feb 08 '16 at 15:28
see http://stackoverflow.com/questions/894860/set-a-default-parameter-value-for-a-javascript-function — mikhail, Feb 08 '16 at 15:29
So do so using non-ES6 JS; not sure what the problem is. Your question was very specific, asking about default parameters in function expressions--by definition that makes it ES6-specific. If your question is actually "how do I do this without ES6 you might want to change the question to reflect that. — Dave Newton, Feb 08 '16 at 15:30
If you want to use that syntax in ES5, use this: https://babeljs.io/ — mikhail, Feb 08 '16 at 15:30
@DaveNewton I am confused. Function Expressions are ES5... so how is my question ES6-specific? — Grateful, Feb 08 '16 at 15:32
`function (arg1 = 'default1', arg2 = 'default2'){}` is ES2015 syntax — mikhail, Feb 08 '16 at 15:33
I guess I am gonna have to do more reading up on the version specifics. Anyway, thank you everybody. — Grateful, Feb 08 '16 at 15:35
@Grateful Function expressions are much older than ES5. Default parameters are ES6. — Dave Newton, Feb 08 '16 at 15:51

Gavriel · Answer 1 · 2016-02-08T15:27:59.513

-1

You can't do this in javascript:

function foo(arg1 = 'default1', arg2 = 'default2') { ; }

See in your console, it gives syntax error. You can do this:

function foo(arg1, arg2) {
   if ("undefined" == typeof arg2) {
       arg2 = "default2";
   }
}

var foo2 = function(arg1, arg2) {
   if ("undefined" == typeof arg2) {
       arg2 = "default2";
   }
}

edited Feb 08 '16 at 15:27

answered Feb 08 '16 at 15:16

Gavriel

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You're not using an ES2015 browser apparently. – Pointy Feb 08 '16 at 15:16
I have Chrome 48. Besides the question was tagged with javascript – Gavriel Feb 08 '16 at 15:17
@Pointy Is chrome not qualified? – Grateful Feb 08 '16 at 15:17
Chrome may not default to supporting it. – Pointy Feb 08 '16 at 15:17
The MDN compatibility table says it's in Chrome 49. – Pointy Feb 08 '16 at 15:21
You most certainly *can* do this in JavaScript, it depends on which *version* you're using. – Dave Newton Feb 08 '16 at 15:26
Sorry, I mean an alternative syntax. I think your suggestion above should work. Thanks. – Grateful Feb 08 '16 at 15:37

Default Parameters in Function Expressions

1 Answers1