how do javascript functions parameter work under the hood

Question

I'm new to programming and am unable to find any good explanation on parameter/arguments, how they work under the hood. For eg:

function changeStuff(a) {
        return a = a * 10;
      }

var num = 10;
console.log(changeStuff(num)); //prints 100
console.log(num); //prints 10

When I call this changeStuff function, how does javascript put variable num into parameter - a? Does it do something like a = num under the hood?

I'm sorry if its a bad or a dumb question.

The argument a will have the value of num which is 10. so the function will calculate 10 * 10 and returns the result. — Thielicious
– Thielicious, Commented Nov 13, 2018 at 15:30
you would need to see compiled code rather than this interpreted code to understand it more, as you don´t even deal with registers here. If it works same way as c++ (probably similar), when calling a function, function parameters are pushed onto current stack. Then the function reads values from the stack according to the amount and function reads/writes those values — juvian
– juvian, Commented Nov 13, 2018 at 15:32
Possible duplicate of stackoverflow.com/questions/13104494/… — Peter Pajchl
– Peter Pajchl, Commented Nov 13, 2018 at 15:33
@Thielicious, so inside javascript engine it is saying num = a? It is just abstracted away from us? — Rohan Sharma
– Rohan Sharma, Commented Nov 13, 2018 at 15:34

juvian · Accepted Answer · 2018-11-13 15:38:48Z

3

You would need to see compiled code rather than this interpreted code to understand it more, as you don´t even deal with registers here.

Assuming it works same way as c++ (probably similar), when calling a function, function parameters are pushed onto current stack. Then the function reads values from the stack according to the amount and function reads/writes those values.

In compiled code, there will not exist such thing as 'a' variable. Only a limited amount of registers are available, so a will actually be one of those. Before assigning that, the value from the register will be pushed onto the stack, so that when function ends, the register can go back to its previous value for the running code that might have been using it before calling the function.

A bit of literature on the subject

answered Nov 13, 2018 at 15:38

juvian

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user10608712 · Accepted Answer · 2018-11-13 15:41:04Z

0

Javascript copies the reference to the function a * 10 to the variable a in this case. So a is a * 10 then and so a * 10 will evaluated and returned.

answered Nov 13, 2018 at 15:41

user10608712

3 Comments

Peter Pajchl Over a year ago

Except 10 is a primitive value and those are passed by value in javascript; if they were passed by reference as you say the num var would be modified by the function.

user10608712 Over a year ago

a * 10 is a function. a will get a reference to the function a * 10. a function is an (complex) object. If you have a complex type, even if it contains primitive types, it will be passed by reference. num will be passed to a in the function a * 10 as value, a in the return statement will get the reference to the function.

Rohan Sharma Over a year ago

thanks, i guess i have to look up pass by value/reference thoroughly, maybe that will help me understand what going on

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