Access local variable inside a callback function

Question

var inner = function() { console.log(x); }

// test 1
(function(cb) { var x = 123; cb(); })(inner);

// test 2
(function(cb) { var x = 123; cb.apply(this); })(inner);

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Answer 1

Fact: when you do var inner = function() { console.log(x); } in your first line, x is not defined. Why? Because, inside your inner function, there's no local declaration of x (which would be done with var x = something). The runtime will then look up in the next scope, that is the global scope. There isn't, also, a declaration of x, so x is also not defined there.

The only places where there is a variable called x are inside each one of your 4 IIFEs following. But inside the IIFEs, each x is a different variable, in a different scope. So, if what you want is to console.log() the x defined inside each IIFE, you are taking the wrong approach.

Keep in mind that, when you define inner, you are capturing the environment inside the function's closure. It means that, whatever value could x have there (in the declaration of the function), would be the available value to the x variable later, when the inner function would be used. The fact that your x there is not defined is only an accessory, and is not what is causing the undesired behavior.

So, what happens is that when you call your inner function inside any of your IIFEs, the x referred to inside the inner function declaration is a captured value of what x had as a value when the function was defined, not the value that x has now in the scope where the function is currently being called. This is what is called lexical scope.

To solve this, you would have to pass the value that you want to console.log() inside the inner function as a parameter to the inner function, as so:

var inner = function(x) { console.log(x); }
// test 1
(function(cb) { var x = 123; cb(x); })(inner);