Javascript - Multiple scope related questions

Question

Please check the code in http://jsfiddle.net/4a42n8g8/. it alerts "undefined". Why? I have 2 questions in particular.

var a = 10;

function x() {
a = 20;
alert(this.a);
}

x();

1) Isn't the variable "a" within function x a global variable since it does not have a var prefix? Or is it that with or without var prefix, any variable within a function is not a global variable?

2) Since var a = 10 is defined outside the function, doesn't it make it a global variable? Or is it that all global variables SHOULD NOT have a var prefix?

try the same code in chrome console and you will get correct answer! — daksh_019
– daksh_019, Commented Dec 24, 2014 at 13:58
for opening chrome console press F12 while chrome is open and then press console in option — daksh_019
– daksh_019, Commented Dec 24, 2014 at 13:59

Ruan Mendes · Accepted Answer · 2014-12-24 14:04:31Z

2

The reason is that jsfiddle is running your code within a function like the following:

window.onload=function(){
var a = 10;

function x() {
    a = 20;
    alert(this.a);
}

x();
}

Therefore, your var a is local to the onload function, but this inside of x points to the global window object

If you change your fiddle settings to No wrap - in <head> (or in <body>), it will work as you expect. http://jsfiddle.net/mendesjuan/4a42n8g8/1/

Question 1

var a is not a global because it's within a function, if it was outside of any functions, it would be global.

Question 2

Same as question one, var a actually is defined in a function.

edited Dec 24, 2014 at 14:04

answered Dec 24, 2014 at 13:56

Ruan Mendes

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12 Comments

Nikhil Pai Over a year ago

So, If the above question was asked in an interview, we would answer it with virtual No wrap, right?

Nikhil Pai Over a year ago

Sp If you can answer and confirm my 2 questions, thanks!

Ruan Mendes Over a year ago

I am not sure what you mean with your interview question. virtual no wrap doesn't make any sense in this context.

Nikhil Pai Over a year ago

I meant, if the above question was asked in an interview(just the snippet), then we should be answering the question keeping in mind that there is no "window.onload...." right?

Nikhil Pai Over a year ago

Or correct me if you think otherwise - 1) any variable outside or within a function, if does not have a var prefix is a global variable.

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