javascript variable is undefined [duplicate]

Answer 1

The var directive is processed on the pre-execution stage, the b becomes local variable before document.write(b);, so at that time, b is undefined.

Note: assign a value to a variable is at the execution time, so you could image your code is like below.

function run(){
    document.write(b);
    var b=1;
}

is the same as:

function run(){
    var b;
    document.write(b);
    b=1;
}

Addtion:

This is why Put every var definition at the top of the function is good practice.