Dynamically create a function with default parameters in JavaScript

Question

One of the features introduced by ECMAScript 6 is the ability to indicate default values for unspecified parameters in JavaScript, e.g.

function foo(a = 2, b = 3) {
    return a * b;
}

console.log(foo());   // 6
console.log(foo(5));  // 15

Now I'm wondering if it's possible to use default parameters also for functions created dynamically with the Function constructor, like this:

new Function('a = 2', 'b = 3', 'return a * b;');

Firefox 39 seems to already support default parameters (see here), but the line above is rejected as a syntax error.

Hmmm, why do you need a string argument instead of just a =2? It would be slightly more difficult to use, isn't it? — TaoPR
– TaoPR, Commented Jul 15, 2015 at 11:43

Bergi · Accepted Answer · 2015-07-15 16:45:10Z

1

Now I'm wondering if it's possible to use default parameters also for functions created dynamically with the Function constructor

Yes, according to the spec this is possible. The parameter arguments are concatenated as always and then parsed according to the FormalParameters production, which includes default values.

Firefox 39 seems to already support default parameters, but the line above is rejected as a syntax error.

Well, that's a bug :-) (probably this one)
You should be able to work around it by using

var fn = Function('return function(a = 2, b = 3) { return a * b; }')();

answered Jul 15, 2015 at 16:45

Bergi

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Florian Wendelborn · Accepted Answer · 2015-07-15 11:52:25Z

0

As new Function is a form of eval anyway you could use the following code for that task:

eval('function bar (a = 2, b = 3) { return a * b; }');

answered Jul 15, 2015 at 11:52

Florian Wendelborn

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1 Comment

Oriol Over a year ago

Probably because it's a function declaration, so eval will return undefined. You should use a function expression instead.

citywall · Accepted Answer · 2015-07-15 11:53:51Z

0

I do not know if that is supposed to work - some one else will have to tell you - but if it continues to be a problem, why not use eval()?

Something in the lines on this should be legal:

var fct1 = eval("(function foo(a = 2, b = 3) { return a * b; })")

answered Jul 15, 2015 at 11:53

citywall

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