Javascript-将函数分配给变量（引用/值）

Question

I am trying to change the definition of a function:

var a = function(){alert('a');};
var b = a;
b = function(){alert('b');};

This results in the variable a keeping it's original assignment ie the function producing alert('a') .

Is there any way of passing around a reference to a javascript function so that I can alter it later?

Answer 1

Would you expect the value of a to change after the following snippet? Snippet in question:

var a = 10;
var b = a;
var b = 20;

No, right? So why would you expect reassigning b to also affect a ?

After line 1 , you have a pointing to a function instance:

After line 2 , you have a new variable b , also pointing to the same function instance. So now you have two variables, both pointing to the same instance:

After line 3 , you have reassigned b to something else (a new function), but a is still pointing to the original function:

You can do what you want by doing something like this:

var func = function() { alert("a"); };

var a = function() { func(); };
var b = a;

func = function() { alert("b"); };

Now calling a() or b() will alert the string b .

Answer 2

Is there any way of passing around a reference to a javascript function so that I can alter it later?

There is no way to do this. Javascript does not have "pointers". It has reference values , and as such, a is a reference to the value of a, not to the memory location of a.

So, for this set of instructions

var a = function(){alert('a');};
var b = a;
b = function(){alert('b');};

this is the progression

//a is stored at some memory location
var a;

//b is stored at some memory location
var b;

//the memory location where a is stored has its value updated
a = function(){alert('a');};

//the memory location where b is stored has its value updated
//from the value stored at a's memory location
b = a;

//the memory location where b is stored has its value updated
b = function(){alert('b');};

Answer 3

You could produce the result you're looking for like this:

var fn = function() { alert('a'); };
var a = function() { fn(); };
var b = a;
fn = function(){ alert('b'); };

This code would produce the desired effect you're looking for because they'll both call fn() and you're changing the common underlying reference.