Javascript if expression evaluation

Question

I was wondering when the Javascript if expression really evaluates to false and when true . When the operator of if false , and - this is true for all interpreters of JS?

I assume the condition is false on

• false
• undefined
• null
• 0

otherwise true . Is this correct for all implementations (verified in the Safari / WebKit console), or is it better for me with explicit verification, for example (typeof a === "undefined") ?

Answer 1

The following values ​​will be calculated as false:

• False
• undefined
• Null
• 0
• NaN
• empty line ("")

https://developer.mozilla.org/en/JavaScript/Guide/Statements#if...else_Statement

Answer 2

If you want to check for the existence of a variable, and a not declared anywhere in your script, typeof a === 'undefined' is the way to go, or you can use if (!window.a) . Otherwise, a ReferenceError is raised.

Your guess seems correct. An empty string and NaN are also evaluated as false (or, like some of them, falsy ). The fact that 0 evaluates to false can be tricky, but it is also convenient in expressions such as while (i--) (if i is 0, it evaluates to false and the while loop stops).