How does Javascript handle function parameters

Question

I recently had an issue with some javascript that goes against every bone of my programming background. Javascript does this often to me, so I'm not that surprised.

I have a function as such...

function x(param1, booleanParam, arrayParam){
    ....
}

I was getting a runtime error saying that arrayParam.length was not defined. On debugging I saw this was true and went to find out why. Turns out I had forgotten a comma in my function call as such...

x(param1, true [arrayJunk]);

The problem I'm having is figuring out why this call was made at all? Why isn't this a compile error, how does Javascript see this and think, "Yeah, that seems like it might work!"

Thanks in advance for any enlightenment you can share!

This is a case where using the Array constructor instead of the literal syntax would have provided a useful SyntaxError, but then you have the potential issue of passing a single numeric argument causing a different silent bug. — user1106925
– user1106925, Commented Jun 4, 2012 at 16:55

SLaks · Accepted Answer · 2012-06-04 16:40:24Z

3

That's an indexing expression.
It's the same syntax as someArray[someIndex].

It will end up passing undefined as the second parameter too, unless arrayJunk happens to be the name of a property of boolean primitives.

answered Jun 4, 2012 at 16:40

SLaks

891k182 gold badges1.9k silver badges2k bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

Shaded Over a year ago

Is true not a reserved word in javascript? How can it try to do an index on a primative?

SLaks Over a year ago

@Shaded: In Javascript, everything is an object. For example, true["toString"]() will return "true".

Ruan Mendes Over a year ago

Not everything is an object; 1, true and "23443" are primitives. Only things where typeof returns "object" are real objects. But everything can be converted into an object, and that's what the JS engine does

hugomg Over a year ago

@SLaks: Well, almost everything. null and undefined don't have properties.

Ruan Mendes Over a year ago

null does not auto convert into an object

|

Ruan Mendes · Accepted Answer · 2012-06-04 16:44:24Z

2

What happens is the following:

JavaScript engine converts true into a Boolean object (not the primitive)
It then tries to access the property name stored in arrayParam from that object
Property doesn't exist, so it returns undefined

If arrayParam was the string "toString", it would return a function object

answered Jun 4, 2012 at 16:44

Ruan Mendes

92.7k31 gold badges161 silver badges225 bronze badges

Comments

JaredPar · Accepted Answer · 2012-06-04 16:41:37Z

0

In this case the expression was being interpreted as an index. Essentially the same as

someArray[42]

So it was being seen as a function call with 2 parameters instead of 3

answered Jun 4, 2012 at 16:41

JaredPar

759k152 gold badges1.3k silver badges1.5k bronze badges

Comments

hugomg · Accepted Answer · 2012-06-04 16:44:26Z

0

Many dynamic languages don't check if you pass too many or too few arguments to a function.

While this can sometimes mask errors, it also allows you to roll your own defalut parameter scheme.

answered Jun 4, 2012 at 16:44

hugomg

70.4k30 gold badges168 silver badges257 bronze badges

Collectives™ on Stack Overflow

How does Javascript handle function parameters

4 Answers 4

7 Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

7 Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related