how does variable use before its declaration?

Question

In Javascript how does variable used before definitions?

console.log("b ", b);

var b;

I want to understand how above statement work in javascript?

Answer 1

b is undefined at code at Question. However, if b were a function, that variable declaration would be "hoisted" see 'Hoisted' JavaScript Variables and b would be defined within console.log() even where not actually defined until the next line.

console.log("b ", b);

function b() {}

Answer 2

JS first check how many variable and functions are going to use and variable are assigned as undefined and assigned at last.

So in the first example you can understand like var b = undefined and then console.log("b ", b); and then b=1;

console.log("b ", b);
var b=1;

console.log("b ", b);

var b=1;
console.log("b ", b);

Answer 3

You can use d and then initialize d it will print undefined.
but if you use d with out any initialization it will return b is not defined Error .

console.log("d",d);

   

 console.log("b "+b);
 var b = "What do you mean ?"    
 console.log("b ",b);