r/learnjavascript • u/hsnice • Jul 19 '21

JavaScript Function Facts

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20

u/[deleted] Jul 19 '21

function declarations aren't the only thing that is hoisted, variable declarations with var are also hoisted.

e.g. the following does not work:

console.log('hello', future());
const future = function() { 
   return ' world'; 
}

1

u/[deleted] Jul 19 '21

Does using var have a higher cost than using let? Does hoisting have a cost?

2

u/[deleted] Jul 19 '21

the only difference is, that you can't redeclare a let/const variable and that var variable are hoisted to the top. There is no cost.

2

u/[deleted] Jul 19 '21

You can redeclare let. I thought the only difference was scoping.

4

u/[deleted] Jul 19 '21

Reassign, yes, but not redeclare.

 let assignment = 3;
 assignment = true;

 let declaring = function() {}
 let declaring = false; // here will be a redeclaration error

-5

u/[deleted] Jul 19 '21

We’ll that’s not really redeclaration is it? You’re assigning 2 variables with the same name?

6

u/[deleted] Jul 19 '21

[removed] — view removed comment

5

u/[deleted] Jul 19 '21

I’ll be honest I thought reassigning was redeclaration.

1

u/[deleted] Jul 19 '21

well it is. the first declaring would be of type 'function' while the second declaring would be a boolean. you can do that in var :

 var a = function() {};
 typeof a // "function"

 var a = false;
 typeof a // "boolean"

3

u/[deleted] Jul 19 '21

Interesting. Thanks for bringing this up!

2

u/oculus42 Jul 20 '21

The declaration and the value are separate. When hoisted, this code would read something like this:

var a;
var a;
a = function() {};
typeof a; // "function"
a = false;
typeof a; // "boolean"

The two instances of var a are declarations of the variable a, and have nothing to do with the values assigned. The function and false are assignments.