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Why is this global variable undefined inside a function if the same global variable is re-declared and defined inside that same function? 

var a = 1;
function testscope(){
 console.log(a, 'inside func');
 //var a=2;
};
testscope();
console.log(a, 'outside func');

output:
1 "inside func"
1 "outside func"

Consider same code where var a = 2; inside function block is uncommented

var a = 1;
function testscope(){
 console.log(a, 'inside func');
 var a=2; 
};
testscope();
console.log(a, 'outside func');

Output
undefined "inside func"
1 "outside func"
Christopher Nuccio
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Raj Rj
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2 Answers2

5

It's because Javascript is not like Java and variable declaration are always pushed up their block. Your second piece of code is strictly equivalent to:

var a = 1;
function testscope(){
 var a;  // <-- When executed, the declaration goes up here
 console.log(a, 'inside func');
 a=2;  // <-- and assignation stays there
};
testscope();
console.log(a, 'outside func');

Output
undefined "inside func"
1 "outside func"
Adrien Brunelat
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-1

because a on the first function refers to the variable a that exists on the function, while a you write after the variable you write. if you want a global variable accessible inside a function that contains the same variable as the global variable you should add this.a. or if you want to access the variable a in the function you have to write the variable before you call it

massam
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