Question

I was wondering when the Javascript if expression actually evaluates to false and when to true. When is the if statement false, and is that true for all JS interpreters?

I guess the condition is false on

• false
• undefined
• null
• 0

otherwise true. Is that correct for all implementations (tested in Safari/WebKit console), or am I better off with explicit checking like (typeof a === "undefined")?

Answer 1

The following values will evaluate to false:

• false
• undefined
• null
• 0
• NaN
• the empty string (“”)

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