在Javascript中，typeof var！==未定义等效于if（var）

Question

In Javascript, is:

if(typeof p_options.data_type !== "undefined") { }

Absolutely equivalent to just doing:

if(p_options.data_type) { }

Are there any edge cases or gotchas?

Answer 1

No . It definitely is not the same.

Imagine if p_options.data_type was false , 0 , "" , or any other "falsey" value. This is very different from undefined .

p_options.data_type = false;

console.log(typeof p_options.data_type !== "undefined"); // true
console.log(p_options.data_type);                        // false