Javascript Function-Pointer Assignment

Question

Consider this javascript code:

var bar = function () { alert("A"); }
var foo = bar;
bar = function () { alert("B"); };
foo();

When running this code I get "A". Is this behavior a part of javascript specification and can I rely on it?

Answer 1

Yes that is expected and by design.

Your question is basically: does foo reference bar as a pointer or reference would in another language?

The answer is no: the value of bar at the time of assignment is assigned to foo.

Answer 2

This is assigning a variable to an unnamed function, not a pointer to a function

Answer 3

Yes, you've created a pointer to the original "A" function. When you reassign bar, you're reassigning it, but you're still leaving any references to the old function alone.

So to answer your question, yes, you can rely on it.

Answer 4

Yes, there's nothing special about the fact that the variables are referring to functions, there's no aliasing involved.

var bar = 1;
var foo = bar;
bar = "something entirely different";
// foo is still 1

Answer 5

Yes, this is the correct behavior.

//create variable bar and assign a function to it
var bar = function () { alert("A"); }
//assign value of bar to the newly created variable foo
var foo = bar;
//assign a new function to the variable bar
//since foo and bar are not pointers, value of foo doesn't change
bar = function () { alert("B"); };
//call the function stored in foo
foo();

Answer 6

Those are not function pointers (and there are no pointers in JS natively). Functions in JS can be anonymous and are first class objects. Hence

function () { alert("A"); }

creates an anonymous function that alerts "A" on execution;

var bar = function () { alert("A"); };

assign that function to bar;

var foo = bar;

assign foo to bar, which is the function "A".

bar = function () { alert("B"); };

rebind bar to an anonymous function "B". This won't affect foo or the other function "A".

foo();

Call the function stored in foo, which is the function "A".

---

Actually in languages where there are function points e.g. C it won't affect foo either. I don't know where you get the idea of getting "B" on reassignment.

void A(void) { printf("A\n"); }
void B(void) { printf("B\n"); }
typedef void(*fptr_t)(void);
fptr_t foo = A;
fptr_t bar = foo;
bar = B;
foo(); // should print "A"

Answer 7

You are assigning the value of an anonymous function to a variable not a pointer.
If you want to play with pointers, you can use objects that are passed by reference, not copy.

Here are some examples:

"obj2" is a reference of "obj1", you change "obj2", and "obj1" is changed. It will alert false.

var obj1 = {prop:true},
    obj2 = obj1;
obj2.prop = false;
alert(obj1.prop);

"prop" points to a property that is not an object, "prop" is not a pointer to this object but a copy. If you change "prop", "obj1" is not changed. It will alert true

var obj1 = {prop:true},
    prop = obj1.prop;
prop = false;
alert(obj1.prop);

"obj2" is a reference to the "subObj" property of "obj1". if "obj2" is changed, "obj1" is changed. It will alert false.

var obj1 = {subObj:{prop:true}},
    obj2 = obj1.subObj;
obj2.prop = false;
alert(obj1.subObj.prop);

Answer 8

(sorry for writing this as an answer; Cannot comment yet.)

@KennyTM Is that really true that there are no pointers in JavaScript? Because it supports Objects.

function Obj(v) {
    this.val = v;
}
a = new Obj(42);
b = a;
a.val = 3;
alert(b.val); // alerts 3

Answer 9

I'm a bit late here but I thought I'd give an answer anyways and flesh something out.

It's best not to think in terms of pointers and memory references when discussing the internals of JavaScript (or ECMAScript) when dealing with the specifications. Variables are environment records internally and are stored and referenced by name, not memory address. What your assignment statement is doing, internally and by design, is looking up the environment record name (either "foo" or "bar") and assigning the value to that record.

So,

var bar = function () { alert("A"); }

is assigning the environment record "bar" the value (anonymous function).

var foo = bar;

internally calls GetValue("bar") which retrieves the value associated with the record "bar" and then associates that value with the record "foo". Hence, afterwards the original value of bar can still be used as it's now associated with foo.

Because JavaScript references by string and not memory address is precisely why you can do things like this:

someObject["someProperty"]

which is looking up the value based on the property name.