CODEWITHSUNDEEP
javascriptfunctionscope
hh54188
hh54188

Reputation: 15626

Function's scope of variables

I execute function like this:

var a = 123;
function f() { 
  alert(a);
  var a = 9;
} 
f();

the result is undefined, why this happened? Why it's not 123?

Upvotes: 3

Views: 197

Answers (2)

Ian
Ian

Reputation: 50905

Your function is actually compiled as:

function f() {
  var a; 
  alert(a);
  a = 9;
}

because of variable hoisting: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Statements/var#var_hoisting

So your function redeclares a as a local variable with the value as undefined, alerts it, and then re-sets its value to 9.

At the time of the alert, its value is undefined because of the hoisting.

Upvotes: 8

Gowtham
Gowtham

Reputation: 12170

If you declare a in a function syntax again that becomes a new variable. If you want to use the previous value 123 then you should not have included the var a = 9 statement, since it creates a new variable. This may explain in detail : What is the scope of variables in JavaScript?

Upvotes: 0

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