为什么`typeof false || undefined`返回“布尔值”

Question

I've just executed the following in console:

typeof false || undefined // "boolean"

While I expected it to return "undefined", since

typeof undefined // "undefined"

Why did it return "boolean"? Isn't the OR operator supposed to return the last argument if previous evaluated to falsey values? So that false || undefined false || undefined returns undefined and typeof is executed against undefined ?

Answer 1

You're not observing operator precedence!

> typeof false || boolean  // (typeof false) || boolean
"boolean"
> typeof (false || boolean)
"undefined"

Answer 2

The OR operator returns the left argument as long as it isn't falsy.

In your example, we've got this:

(typeof false) || undefined

typeof false returns "boolean" , which isn't falsy, so it is returned rather than undefined .

Answer 3

Consider the code :

typeof false || undefined

typeof false will return "boolean" , so it will become "boolean" || undefined "boolean" || undefined

The final output will be "boolean"