pre-defined parameters

Question

I want to be able to do this in JavaScript:

function myFunction(one, two = 1) {
     // code
}

myFunction("foo", "2");

myFunction("bar");

I tried this and it doesn't work. I don't know how to call this type of parameters, could somebody point me in the right direction?

Thanks.

Answers involving the `two = two || x;` pattern should be understood as risky, depending on your situation. It means to override *any* "falsy" value of the parameter with the default value, which may or may not be appropriate. — Pointy, Oct 28 '12 at 17:10

score 3 · Accepted Answer · edited May 23 '17 at 12:00

3

function foo(a, b)
 {
   a = typeof a !== 'undefined' ? a : 42;
   b = typeof b !== 'undefined' ? b : 'default_b';
   //...
 }

edited May 23 '17 at 12:00

Community

answered Oct 28 '12 at 17:08

Mihai Matei

definite duplicate...but you *do* want to check explicitly for undefined as in this answer – Matt Whipple Oct 28 '12 at 17:09
Because `typeof` returns the type of the parameter you gave.. if `a` is a string then `string` is returned and if there is no parameter then the typeof will return `undefined` – Mihai Matei Oct 28 '12 at 17:15
If you think this is a duplicate, you should vote to close or flag it as such. Don't copy the answer here. – Felix Kling Oct 28 '12 at 18:00

Anurag Uniyal · Answer 2 · 2012-10-28T17:24:23.867

2

function myFunction(one, two) {
     two = two || 1
}

To be more precise e.g. it may not work when two is zero, check for null or undefined e.g.

if (typeof two === "undefined") two = 1

edited Oct 28 '12 at 17:24

answered Oct 28 '12 at 17:08

Anurag Uniyal

score 2 · Answer 3 · answered Oct 28 '12 at 17:09

2

Use this :

function myFunction(one, two) {
   if (typeof two == 'undefined') two = 1;
   ...
}

Beware not to do the common mistake

two = two || 1;

because this wouldn't allow you to provide "" or 0.

answered Oct 28 '12 at 17:09

Denys Séguret

score 0 · Answer 4 · answered Oct 28 '12 at 17:08

0

Try:

if ( typeof two === "undefined" ) two = 2;

or

two = typeof two !== "undefined" ? two : 2;

Undefined arguments will have the value undefined, which is a "falsy" value. We can test this falsyness, and change the value accordingly.

answered Oct 28 '12 at 17:08

0x499602D2

4 Answers4