How the statement if is evaluated in javascript

Question

Possible Duplicate:
Is there a standard function to check for null, undefined, or blank variables in JavaScript?

How is a condition inside the if statement evaluated to true or false in JavaScript?

Answer 1

undefined, null, "", 0, NaN, and false are all "falsey" values. Everything else is a "truthy" value.

If you test a "falsey" value, the condition is false. E.g.:

var a = 0;

if (a) {
    // Doesn't happen
}
else {
    // Does happen
}

If you test a "truthy" value, the condition is true:

var a = 1;

if (a) {
    // Does happen
}
else {
    // Doesn't happen
}

Answer 2

Whatever the result of the condition expression is, it is converted to a Boolean by ToBoolean:

1. Let exprRef be the result of evaluating Expression.
2. If ToBoolean(GetValue(exprRef)) is true, then
   Return the result of evaluating the first Statement.
3. Else,
   Return the result of evaluating the second Statement.

For each data type or value, the conversion rules to a Boolean are clearly defined. So, for example, undefined and null are both converted to false.