What is the scope of variables in JavaScript?

1731

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?

Javascript
asked 9 years ago
Sunny Solu

1Answer

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.

A globally-scoped variable

var a = 1;

// global scope
function one() {
  alert(a);
}

Local scope

var a = 1;

function two(a) {
  alert(a);
}

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

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

var a = 1;

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

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

Intermediate: Object properties

var a = 1;

function five() {
  this.a = 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);
  };
})();

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;

Altogether

// These will print 1-8
one();
two(2);
three();
four();
alert(new five().a);
six();
alert(new seven().a);
alert(new seven().b);

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); 
})();
```
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.

answered 8 years ago
B Butts

What is the scope of variables in JavaScript?

Javascript

1Answer

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