What is the purpose of the var keyword and when should I use it (or omit it)?

user_114916 0
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NOTE: This question was asked from the viewpoint of ECMAScript version 3 or 5. The answers might become outdated with the introduction of new features in the release of ECMAScript 6.

What exactly is the function of the var keyword in JavaScript, and what is the difference between

var someNumber = 2;
var someFunction = function() { doSomething; }
var someObject = { }
var someObject.someProperty = 5;

and

someNumber = 2;
someFunction = function() { doSomething; }
someObject = { }
someObject.someProperty = 5;

?

When would you use either one, and why/what does it do?

ecmascript-5 keyword javascript
19 Answers

As someeone trying to learn this this is how I see it. The above examples were maybe a bit overly complicated for a beginner.

If you run this code:

var local = true;
var global = true;


function test(){
  var local = false;
  var global = false;
  console.log(local)
  console.log(global)
}

test();

console.log(local);
console.log(global);

The output will read as: false, false, true, true

Because it sees the variables in the function as seperate from those outside of it, hence the term local variable and this was because we used var in the assignment. If you take away the var in the function so it now reads like this:

var local = true;
var global = true;


function test(){
  local = false;
  global = false;
  console.log(local)
  console.log(global)
}

test();

console.log(local);
console.log(global);

The output is false, false, false, false

This is because rather than creating a new variable in the local scope or function it simply uses the global variables and reassigns them to false.

user_23526480

Besides scopes issue, some folks also mention hoisting, but no one gave an example. Here's one for global scope:

console.log(noErrorCase);
var noErrorCase = "you will reach that point";

console.log(runTimeError);
runTimeError = "you won't reach that point";

user_38729760
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