Scope Issues With Setting Javascript Function

Question

I am in a position where I need to "update" a function that exists in another javascript file. The file looks like this:

function jf(){
    alert('1');
}

//call jf periodically
 jf();

The second js file, which is loaded after looks like this:

console.log(jf);
console.log(window.jf);
var func=function(){
  alert('2');  
};
jf=func;
window.jf=func;

The first log successfully returns the original jf method, the second doesnt. The first set seems to set the local variable jf, and the second does basically nothing. Is there a way to achieve this functionality?

Your console.log(window.jf) executes before the window.jf=func call and so it couldn't possibly output the jf method. — Bill
– Bill, Commented Jul 11, 2012 at 4:07
second is undefined. The original function jf() executes long before either console.log. The window log checks to see if that function exists in the window scope. — Dested
– Dested, Commented Jul 11, 2012 at 4:08
Well the second jf = func; should "update" jf. What happens if you call jf(); after updating it? — Anthony Mills
– Anthony Mills, Commented Jul 11, 2012 at 4:13

Community · Accepted Answer · 2017-05-23 12:27:53Z

2

According to Javascript closures - behavior of overridden functions from the global scope

var done = and function done do basicaly the same thing. They will shadow the outer definition in the inner scope but they will not replace it on the outer scope.

This means you can only override your initial definition of function jf() if you are in the same execution context. Otherwise, replace function jf(){ ... with window.jf = function(){...

Also, running your tests in an inspector console might help.

edited May 23, 2017 at 12:27

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answered Jul 11, 2012 at 4:17

Silviu-Marian

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8 Comments

Dested Over a year ago

Yeah... This is what I was looking for. And for the record, there is no way to simulate this context? The context of a completely separate js file that is.

Silviu-Marian Over a year ago

Unfortunately that's the whole point of scopes & contexts, to isolate one part of the program from another.

Dested Over a year ago

shuuccks. I wonder if I could just replace the script tag.... Thank you very much grigore, very informative.

RobG Over a year ago

@GRIGORE-TURBODISEL—I think you're confused. Each script element doesn't have a new scope, functions and windows do. The OP's code is all running in the one global context. The issue is one of execution order—assignment of a new function expression to (global) jf occurs after calling jf, so clearly the new function hasn't been assigned yet so it isn't called. Incidentally, jf and window.jf refer to exactly the same property of the global object.

Silviu-Marian Over a year ago

@RobG the symptoms OP described suggest that jf() wasn't defined globally. If it was, the symptoms would've been different and it's all been covered in the reference I gave. And you should also see stackoverflow.com/questions/9941736/…

|

Anthony Mills · Accepted Answer · 2012-07-11 04:07:49Z

1

First, use variables:

var jf = function () {
    alert('1');
};

jf();

Then the second bit should work fine:

var func = function () {
    alert('2');
};

jf = func;
jf();

answered Jul 11, 2012 at 4:07

Anthony Mills

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2 Comments

Dested Over a year ago

In a perfect world, yes. Unfortunately that's not an option. I'm not asking for best practices, just wondering if this one case is possible.

RobG Over a year ago

@Dested—the new value of func isn't assigned until after you attempt to call it. It has nothing at all to do with scope and everything to do with execution order. It is all running in the one global scope.

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