Wednesday 31 May 2017

function - What is the scope of variables in JavaScript?



What is the scope of variables in javascript? Do they have the same scope inside as opposed to outside a function? Or does it even matter? Also, where are the variables stored if they are defined globally?


Answer



I think about the best I can do is give you a bunch of examples to study.
Javascript programmers are practically ranked by how well they understand scope.

It can at times be quite counter-intuitive.




  1. A globally-scoped variable



    // global scope
    var a = 1;

    function one() {
    alert(a); // alerts '1'

    }

  2. Local scope



    // global scope
    var a = 1;

    function two(a) { // passing (a) makes it local scope
    alert(a); // alerts the given argument, not the global value of '1'
    }


    // local scope again
    function three() {
    var a = 3;
    alert(a); // alerts '3'
    }

  3. Intermediate: No such thing as block scope in JavaScript (ES5; ES6 introduces let and const)



    a.




    var a = 1;

    function four() {
    if (true) {
    var a = 4;
    }

    alert(a); // alerts '4', not the global value of '1'
    }



    b.



    var a = 1;

    function one() {
    if (true) {
    let a = 4;
    }


    alert(a); // alerts '1' because the 'let' keyword uses block scoping
    }


    c.



    var a = 1;

    function one() {

    if (true) {
    const a = 4;
    }

    alert(a); // alerts '1' because the 'const' keyword also uses block scoping as 'let'
    }

  4. Intermediate: Object properties



    var a = 1;


    function Five() {
    this.a = 5;
    }

    alert(new Five().a); // alerts '5'

  5. Advanced: Closure



    var a = 1;


    var six = (function() {
    var a = 6;

    return function() {
    // JavaScript "closure" means I have access to 'a' in here,
    // because it is defined in the function in which I was defined.
    alert(a); // alerts '6'
    };
    })();


  6. Advanced: Prototype-based scope resolution



    var a = 1;

    function seven() {
    this.a = 7;
    }

    // [object].prototype.property loses to

    // [object].property in the lookup chain. For example...

    // Won't get reached, because 'a' is set in the constructor above.
    seven.prototype.a = -1;

    // Will get reached, even though 'b' is NOT set in the constructor.
    seven.prototype.b = 8;

    alert(new seven().a); // alerts '7'
    alert(new seven().b); // alerts '8'





  7. Global+Local: An extra complex Case



    var x = 5;

    (function () {
    console.log(x);
    var x = 10;

    console.log(x);
    })();


    This will print out undefined and 10 rather than 5 and 10 since JavaScript always moves variable declarations (not initializations) to the top of the scope, making the code equivalent to:



    var x = 5;

    (function () {
    var x;

    console.log(x);
    x = 10;
    console.log(x);
    })();

  8. Catch clause-scoped variable



    var e = 5;
    console.log(e);
    try {

    throw 6;
    } catch (e) {
    console.log(e);
    }
    console.log(e);


    This will print out 5, 6, 5. Inside the catch clause e shadows global and local variables. But this special scope is only for the caught variable. If you write var f; inside the catch clause, then it's exactly the same as if you had defined it before or after the try-catch block.



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