global variable returns undefined when used in a function

Question

When I define the variable outside of a func, it returns undefined. I couldn't find out the reason why.

document.write(myFunc());

var x = 1;
function myFunc() {
    return x;
}

output: undefined

However if I define variable inside the func, it works.

document.write(myFunc());

function myFunc() {
    var x = 1;
    return x;
}

output: 1

You are calling myFunc before the value has been assigned to x. — RobG
– RobG, Commented Mar 11, 2016 at 1:26

RobG · Accepted Answer · 2016-03-11 01:31:21Z

1

You have fallen foul of a common misconception. Variable and function declarations are processed before any code is executed, however assignments occur in sequence in the code. So your code is effectively:

// Function declarations are processed first
function myFunc() {
    return x;
}

// Variable declarations are processed next and initialised to undefined
// if not previously declared
var x;

// Code is executed in sequence
document.write(myFunc());

// Assignment happens here (after calling myFunc)
x = 1;

answered Mar 11, 2016 at 1:31

RobG

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2 Comments

Behram Over a year ago

Thanks a lot for your response. However, It makes too complicated to assign a value to x = 1 at the end of execution. Wouldn't it be better to assign it when defining the value?

RobG Over a year ago

@BehramBeldagli—yes, of course, I'm just showing the effect of processing your code. If you want it to work, then put the initialiser for x at the top so the assignment happens first (or at least before myFunc is called).

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