Why do some non-empty strings evaluate to "false" in JavaScript?

Question

According to this table in the ECMAScript standard, string values that have length 0 should be evaluated as boolean false.

How come, then, these statements evaluate to true?

"\t" == false
" " == false
"\n" == false
"     " == false

All those strings have a length greater than 0. For example:

Not falsey

While I understand that "0" evaluates to false because it can be coerced to a numeric 0, I can't explain why these strings are falsey. What's going on?

(Obviously I can use === for a strict comparison, but in this case in my code, I need the loose comparison, however I wasn't expecting a non-empty string to be considered falsey.)

probably duplicated with this page of stackoverflow:stackoverflow.com/questions/5634372/… — Reza
– Reza, Commented Nov 19, 2014 at 20:09
@vagueness Yeah, pretty much. It didn't show up in my searches, unfortunately. — Matt
– Matt, Commented Nov 19, 2014 at 20:11

Felix Kling · Accepted Answer · 2014-11-19 21:20:15Z

10

You are using loose comparison, which performs type conversion. Whenever you compare against a Boolean, both values are actually converted to numbers (spec, steps 7 and 5). false is 0 and (surprisingly!)every string containing only white space characters is converted to 0 as well (when converted to a number) (spec):

The MV of StringNumericLiteral ::: StrWhiteSpace is 0.

I wasn't expecting a non-empty string to be considered falsey

Comparing a value against a Boolean is very different from converting a value to a Boolean. "Falsy" means that the value is converted to false when converted to a Boolean. However, again, in your case the values are converted to numbers first.

Example:

Number("    ") // 0 ( == Number(false))
// vs
Boolean("    ") // true

edited Nov 19, 2014 at 21:20

answered Nov 19, 2014 at 20:06

Felix Kling

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1 Comment

Felix Kling Over a year ago

@RobG: I only pointed out where Booleans are converted, but you are right, the way I phrased the sentence before creates other expectations. Updating :)

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