Results of "truthy" and "falsey" is confusing in JavaScript

Question

I am learning JavaScript basics. I am confused about the results of "truthy" and "falsey". Can anyone tell the reason for this?

myVariable = undefined ? "truthy" : "falsey";

The result is "falsey".

myVariable = typeOf someUndefinedVariable ? "truthy" : "falsey";

The result is "truthy".

I expected "falsey" instead for the second result. Why did I get "truthy"? I am really confused.

Answer 1

typeof returns the type name of the value, which is always a non-empty string. A non-empty string is truthy.

> typeof someUndefinedVariable
"undefined"
> typeof (typeof someUndefinedVariable)
"string"
> Boolean("undefined")
true

See also All falsey values in JavaScript

Answer 2

First, I'll explain what happened in the code you've shown, just to be sure you understand it. Maybe you already understand the first half, but since you say you are very confused, I will assume you don't.

myVariable = undefined ? "truthy" : "falsey";
// result: "falsey"

Here you are looking at a value, undefined, and asking a question: is it truthy or falsey? Based on that you choose a string. Then you store that string into myVariable. It is effectively the same as this code:

var myVariable;
if (undefined) {
    myVariable = 'truthy';
} else {
    myVariable = 'falsey';
}

Since undefined is always falsey, at the end, myVariable contains the string 'falsey'.

Here is your second snippet of code:

myVariable = typeOf someUndefinedVariable ? "truthy" : "falsey";
// result: "truthy"

First, I must point out that typeOf is not a valid function or operator in any Javascript runtime I know of. I will assume that was a typo, and you meant to use typeof instead. The code above, as written, should throw a syntax error. Corrected, we have...

myVariable = typeof someUndefinedVariable ? "truthy" : "falsey";
// result: "truthy"

And for clarity, let's rewrite it as an if/else:

var myVariable;
if (typeof someUndefinedVariable) {
    myVariable = 'truthy';
} else {
    myVariable = 'falsey';
}

As another answerer has pointed out, typeof will always return, as a string, the type of the symbol you give to it.

typeof(someUndefinedVariable);
"undefined"

And since all strings except the empty string are truthy, you get truthy back.

There are only 7 falsey values in Javascript.

false               Boolean false
undefined           If a symbol is not defined at all
null                A defined symbol, but it has no value inside
0                   Numeric positive 0
-0                  Numeric negative 0
'' or ""            Empty string
NaN                 Magic value "Not a Number"

All other values in Javascript are truthy.

• any string except empty string
• any number except 0
• any array, including an empty array
• any object, including an empty object
• functions
• etc ...