Declaring a Javascript variable twice in same scope - Is it an issue?

Question

Would the following code cause any issues?:

var a = 1; var a = 2; 

My understanding is that javascript variables are declared at the start of the scope. For example:

var foo = 'a'; foo = 'b'; var bar = 'c'; 

Is processed as:

var foo; var bar; foo = 'a'; foo = 'b'; bar = 'c'; 

Therefore would my initial code snippet become:

var a; a = 1; a = 2; 

Or would it become:

var a; var a; a = 1; a = 2; 

I understand that declaring a javascript variable twice in the same scope isn't good practice, but I'm more interested in the impact of doing this.

Answer 1

As you said, by twice of more the same var, JavaScript moves that declaration to the top of the scope and then your code would become like this:

var a; a = 1; a = 2; 

Therefore, it doesn't give us any error.

This same behaviour occurs to for loops (they doesn't have a local scope in their headers), so the code below is really common:

for (var i = 0; i < n; i++) {     // ... } for (var i = 0; i < m; i++) {     // ... } 

That's why JavaScript gurus like Douglas Crockford suggest programmers to manually move those declarations to the top of the scope:

var i;    // declare here for (i = 0; i < n; i++) {    // initialize here...     // ... } for (i = 0; i < m; i++) {    // ... and here     // ... }