Javascript-了解范围链的参考优先级

Question

I would like to better understand the scope chain the hopefully closure better and so the following code:

function greet(whattosay){

    return function(name) {
        whattosay = "not hi"; //change the hi message
        console.log(whattosay + ' ' + name);
        function insideAnon() {
            console.log(whattosay);
        }

    }


}
var sayHi = greet('Hi'); //get in return the anon function
sayHi("John"); //print not hi John on the console

I have recently learned that every "variable environment" is actually an array or an object with properties, and so each function has a reference to it's parent's function variable environment object/array. But my question is, what exactly is the scope chain?(I know that it is supposedly going "down" up until the global level), My current perception (Not sure) is that each child has it's parent and grandparent (and so on) variable environment references inside of it, and so it first check it's parent's variable environment object, if a variable is not there it looks at it's grandparent's reference and so on. Or is it more like - Check the parent variable environment object reference, if not there that parent checks his parent's reference and so on. I HOPE i was very clear at what I was trying to say - How it really goes down the "chain".

Answer 1

You can get references about scope chains from this question:

Scope Chain in Javascript

Answer 2

Each function creates a scope object that contains the arguments for that function and the local variables defined in that function.

Since Javascript uses lexical scoping, that means that a given piece of code can access variables defined within the current function or within any parent function. Javascript makes this work by maintaining a chain of scope objects so each scope has a reference to its parent scope.

When a given piece of code accesses a variable like:

foo = 1;

Javascript, looks in the current scope object to see if something named foo exists there. If so, that variable is used. If not, it gets the reference to the parent scope, checks that scope for something named foo and continues this process up the parent chain until it gets to the global scope and thus can go no further.

So, in this example:

function one() {
    var foo = 2;

    function two() {
        var fee = 3;

        function three() {
            foo = 1;
        }
        three();
    }
    two();
}
one();

The function one() will execute and define foo and two in its scope. It will then call two() which will execute and define fee and three in its scope. It will then call three() which will execute and attempt to change the value of foo . It will first look in the scope object belonging to three , but it will not find anything named foo there so it will go to the parent scope of three which will be the two scope object. It will not find a variable named foo in that scope object so it will go to the parent object of two which will be the scope object of one . There it will find a variable named foo so it will change the value of that variable.

---

On thing that is different about Javascript scope objects compared to something like a C++ stack frame is that they can continue to live after the function that created them has finished executing. This is a very common structure in Javascript and is often referred to as a closure. Here's a simple example of that:

function initializeCntr() {
     var cntr = 0;
     document.getElementById("test").addEventListener("click", function(e) {
         ++cntr;
         document.getElementById("clickCnt").innerHTML = cntr;
     });
}

initializeCntr();

In this short code example, there are three scopes present, the global scope (that contains the initializeCntr symbol), the initializeCntr scope that contains the cntr variable and the scope that belongs to the anonymous function that is set as the event handler.

When you call initializeCntr() , it creates a function scope object that contains the variable cntr . It then runs the addEventListener() method and passes a reference to the inline anonymous event handler for addEventListener() . The initializeCntr() method then finishes execution. But, because the inline anonymous function passed as the event handler contains a reference to the cntr variable in the initializeCntr scope object, that scope object is NOT garbage collected even though initializeCntr() has finished executing. Instead, it is kept alive because of the lasting reference to the cntr variable in it. When that click event handler is then called sometime in the future, it can use that cntr variable. In fact, the initalizeCntr scope will be kept alive as long as the event handler is active. If the event handler is removed or the DOM object it is attached to is removed, then and only then would the initializeCntr scope object be eligible for garbage collection and eventually get freed.

Some JS implementations are smart enough to not necessarily retain the entire scope object and everything in it, but instead to only retain the elements of the scope object that are specifically referenced in child scopes that are still active. So, if there were other variables next to cntr that were not used in the event handler, those other variables could be freed.

FYI, this longer lasting scope is called a closure. Closures are very useful concepts in Javascript and something not available in a language like C++ (because they rely on garbage collection).