JQuery Javascript undefined paradox

Question

Could someone please explain why the alert is triggered and states that the variable is undefined, when the if test says that it is defined?

var some_var;
if(typeof some_var !== undefined){
 alert(some_var); 
}

Fiddle

• Icepickle: typeof some_var !== 'undefined' as typeof something gives a string value
• MinusFour: Why do you need typeof for that? Why not some_var !== undefined?
• Ally: @MinusFour, that's what I used in the first place, but the result was not as expected. So I then used typeof. I have used undefined so many times before, but @Tushar's explanation of strict inequality taught me a valuable lesson.

Answer 1

typeof always returns string.

As you're using strict inequality, the condition evaluates to true.

You can either

1. use undefined as string
2. use != for inequality

Updated Fiddle

var some_var;
if (typeof some_var !== 'undefined') {
 alert(some_var);
}

As suggested by @MinusFour, you can use

if (some_var !== undefined) {

• Ally: Thank you for a great explanation.