Javascript default parameter

Question

I was reading the Three.js code when a silly question arose: Is there any difference between the codes below?

frameHeight = frameHeight !== undefined ? frameHeight : 24;

and

frameHeight = frameHeight || 24;

(frameHeight is a parameter of the function)

Thanks

Not a javascript expert, but I believe the first will only return 24 if frameHeight is undefined where as the second will return 24 if frameHeight has any "falsey" value, like null, '', etc. — sellmeadog
– sellmeadog, Commented Aug 15, 2012 at 17:32
They are different, but if(frameHeight===undefined)frameHeight=24; IS equivalent but it is more readable and performs better since its value will only change when needed — ajax333221
– ajax333221, Commented Aug 15, 2012 at 17:48

jbabey · Accepted Answer · 2012-08-15 17:32:17Z

9

Yes, they are different.

frameHeight = frameHeight || 24;

This will coerce frameHeight to a boolean value. If it is 0, '', false, null, undefined, or NaN it will be false and frameHeight will be defaulted to 24.

frameHeight = frameHeight !== undefined ? frameHeight : 24;

This will explicitly check if frameHeight is not undefined and ONLY for undefined will it default it to 24.

answered Aug 15, 2012 at 17:32

jbabey

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Comments

The Internet · Accepted Answer · 2012-08-15 17:31:50Z

2

frameHeight = frameHeight || 24;

^ Will do a null check as well. Will also do a check for 0, false, empty string ('') NaN and undefined

frameHeight = frameHeight !== undefined ? frameHeight : 24;

^ Will just check for undefined.

answered Aug 15, 2012 at 17:31

The Internet

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Comments

jfriend00 · Accepted Answer · 2012-08-15 18:13:36Z

2

Yes, there is a difference and that difference can be significant depending upon the circumstances.

frameHeight = frameHeight || 24

will assign 24 to frame if frameHeight is initially ANY falsey value such as "", 0, null, undefined, NaN or false.

Whereas:

frameHeight = frameHeight !== undefined ? frameheight : 24

will only assign it 24 if the initial value is exactly undefined.

So, of possible significance in this particular function, the second method will allow you to pass 0 for the frameHeight to set a zero height, but the first method will not because it will override that 0 to 24.

edited Aug 15, 2012 at 18:13

answered Aug 15, 2012 at 17:34

jfriend00

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Comments

Diode · Accepted Answer · 2012-08-15 17:34:30Z

0

frameHeight = frameHeight || 24;

fails for frameHeight = 0 works for frameHeight = null

frameHeight = frameHeight !== undefined ? frameHeight : 24;

fails for frameHeight = null works for frameHeight = 0

answered Aug 15, 2012 at 17:34

Diode

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6 Comments

The Internet Over a year ago

It will return 24 in both the cases of null and 0 for frameHeight = frameHeight || 24.

Diode Over a year ago

fails in the sense - In first case if value is 0 it will be reset to 24, but 0 is valid. If value is null it will be reset to 24, because null is invalid value. So fails for 0 works for null.

Diode Over a year ago

In second case, if value is 0 it won't be reset, because 0 is valid. If value is null it won't be reset, but null is invalid value. So works for 0, fails for null.

The Internet Over a year ago

In my chrome dev tools when I do this: var a = 0; var c = a || 24. c equals 24.

Diode Over a year ago

Yes, but it is wrong logically because if frameHeight is 0, it should not change it to 24.

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