Question

131

JavaScript local and global variable confusion

rated 0 times [ 136] [ 5] / answers: 1 / hits: 22026 / 11 Years ago, fri, november 1, 2013, 12:00:00

I am new to JavaScript and I was doing some practices on local and global variable scopes. Following is my code(fiddle):

var myname = initial

function c(){

    alert(myname);

    var myname = changed;

    alert(myname);

}

c();

When the first alert is called, it is showing myname as undefined. So my confusion is why I am not able to access a global instance of myname and if I don't define myname within the function then it will work fine.

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clifford

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3 Years ago

answered 11 Years ago trevionbronsonr · Accepted Answer

In JavaScript, the variable declarations are automatically moved to the top of the function. So, the interpreter would make it look more like this:

var myname = initial

function c(){

    var myname;

    // Alerts undefined

    alert(myname);

    myname = changed;

    // Alerts changed

    alert(myname);

}

c();

This is called hoisting.

Due to hoisting and the fact that the scope for any variable is the function it's declared in, it's standard practice to list all variables at the top of a function to avoid this confusion.