CODEWITHSUNDEEP
javascriptglobal-variables
user1690995
user1690995

Reputation:

Declaring Empty Variables to make them global

In a JavaScript environment can I declare a variable before a function to make the variable reachable on a global level. For instance:

var a;
function something(){
  a = Math.random()
}

Will this make "a" a global variable?

or is using...

var a = function(){
  var b = Math.random();
  return b;
}
document.write(a())

Really the only way to do it?

Is there a way to make "b" global other than calling the function "a()"?

Upvotes: 2

Views: 5601

Answers (3)

Joseph Silber
Joseph Silber

Reputation: 219938

There are basically 3 ways to declare a global variable:

  1. Declaring it in the global scope, outside of any function scope.
  2. Explicitly assigning it as a property of the window object: window.a = 'foo'.
  3. Not declaring it at all (not recommended). If you omit the var keyword when you first use the variable, it'll be declared globally no matter where in your code that happens.

Note #1: When in strict mode, you'll get an error if you don't declare your variable (as in #3 above).

Note #2: Using the window object to assign a global variable (as in #2 above) works fine in a browser environment, but might not work in other implementations (such as nodejs), since the global scope there is not a window object. If you're using a different environment and want to explicitly assign your global variables, you'll have to be aware of what the global object is called.

Upvotes: 10

Zorayr
Zorayr

Reputation: 24912

Before you think of making a variable global scope, you should consider JavaScript global namespace pollution. The more global variables you declare, the more it is likely that you application will get into conflict with another application's namespace and break. As such, it is highly important minimize the number of global variables.

Upvotes: 0

Bergi
Bergi

Reputation: 664503

Will this make "a" a global variable?

A var declaration makes the variable local to the enclosing scope, which is usually a function one's. If you are executing your code in global scope, then a will be a global variable. You could as well just omit the var, then your variable would be implicitly global (though explicit declaration is better to show your intention).

Is there a way to make "b" global other than calling the function "a()"?

Your b variable is always local to the function a and will never leave it, unless you remove the var.

Upvotes: 2

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