此Javascript代码段如何显示“ false”？

Question

Looking at the code below, I don't understand how (typeof a !== 'undefined') works. I understand that variable "a" is not inside the IIFE, so it is undefined outside of it. But I don't understand how the code prints out 'false'

(function(){
   var a = 3;   
})();

console.log("a defined? " + (typeof a !== 'undefined'));
//prints false- but how and why?

If I remove the typeof and just execute

console.log("a defined? " + (typeof a !== 'undefined')); 
Uncaught ReferenceError: a is not defined

I get an error saying Uncaught ReferenceError: a is not defined. So how is it that it runs fine when typeof is present- and it errors when typeof is removed? That's my confusion

Answer 1

As you say, a doesn't exist outside the IFFE. It is therefore undefined in the different scope.

//Enter the following in your console.
typeof a   // --> "undefined"

(function(){
   var a = 3;   
})();

typeof a   // --> "undefined" (Still)

The issue may come when you're checking the type, and perhaps misunderstanding the Boolean evaluation (it's kind of a double negative). typeof a === 'undefined' is true , therefore, typeof a !== 'undefined' is false .

Answer 2

Firstly the post has the presence or absence of "typeof" interchanged - it should be inserted in the first example and removed in the second. But anyway.

So how is it that it runs fine when typeof is present- and it errors when typeof is removed?

The answer is in the nature and design of the typeof operator.

Whether in strict mode or not, reading the value of an undeclared variable to get its value generates a "Reference Error: a is not defined" except when using the typeof operator .

The typeof operator was designed to allow testing for an undeclared variable and returns the primitive value undefined if it has not been declared. There really is no other means of performing this test without thowing an error.

The test result is still ambiguous if the reason for the test is not clearly understood: the type of a declared variable with value undefined also returns "undefined".

So in the first(?) example if you test typeof a !== 'undefined , a is undefined/undeclared outside the IIFE, typeof a returns "undefined" which compared to itself in an inequality test returns false.

When tested without 'typeof` you get the reference error.