Why is 'false' truthy in javascript?

问题

I understand that an empty string is falsy in javascript and a not-empty string is truthy in javascript.

However, why is 'false' truthy in javascript, is there anything explicit in the specification? Is it a performance issue or are there situations where you would want the string 'false' to represent true?

回答1:

Replying to the last part of your question:

Are there situations where you would want the string 'false' to represent true?

Let's consider I am testing for empty strings in my user input. To achieve that, I issue:

if (!theInput) {
    // Do something.
}

Now do I want that condition to be true if the user enters false in the text box? Of course, I don't.

回答2:

is there anything explicit in the specification?

Yes:

The result is false if the argument is the empty String (its length is zero); otherwise the result is true.

回答3:

In Javascript any string that is not empty is truthy.

So, when evaluating any non-empty string results in true even when the string itself is 'false'.

You can read more about truthy and falsy values.

If you want to check a string for truthness, you can check it's length.

var val = str.length > 0;

回答4:

The value of a non-empty string is always true.

Boolean(false) returns false

Boolean('false') returns true

回答5:

It's by definition. It's a string and it is handled as such. No matter the meaning of the string.

If your logic would be applied. Then how about following examples:

"1+3==2"
"humans have four eyes"

Are they also falsy?

来源：https://stackoverflow.com/questions/32720503/why-is-false-truthy-in-javascript