Мыслепомойка

JavaScript Casting to Number Quiz

Today I was playing with JavaScript again, and as result a small quiz was born. Hope you’ll enjoy.

1. var i;
   alert(++i);
   

   1. Error
   2. 1
   3. NaN
   4. undefined

2. var i = null;
   alert(++i);
   

   1. Error
   2. 1
   3. NaN
   4. undefined

3. var i = {};
   alert(++i);
   

   1. Error
   2. 1
   3. NaN
   4. undefined

4. var i = "";
   alert(++i);
   

   1. Error
   2. 1
   3. NaN
   4. undefined

5. var i = "0";
   alert(++i);
   

   1. Error
   2. 1
   3. NaN
   4. undefined

6. var i = "a";
   alert(++i);
   

   1. Error
   2. 1
   3. NaN
   4. undefined

7. var i = false;
   alert(++i);
   

   1. Error
   2. 1
   3. NaN
   4. undefined

Answers:

1. NaN (undefined + number == NaN)
2. 1 (null is casted to 0)
3. NaN (object + number == NaN, except nulls)
4. 1 (empty string is casted to 0)
5. 1 (string “0” is casted to 0)
6. NaN (string with not a number is casted to NaN)
7. 1 (booleans are casted to numbers: false to 0 and true to 1)

JavaScript magic - Hoisting

Recently I’ve faced with an interesting quiz:

var a = "a";
function b() {
    if (false) {
        var a = "b";
    }
    alert(a);
}

b();

What would be alerted?

1. “a”
2. “b”
3. “undefined”
4. Error

Actually that code is equal to the following:

var a = "a";
function b() {
    var a; // here we declare new function scope variable a,
            // and it's default value is undefined
    if (false) {
        a = "b"; // due to the if-condition this code will be never executed
    }
    alert(a); // so we alert the local variable's value
}

b();

The right answer is “undefined”.