string to boolean conversion behaviour

Question

Are all non empty strings evaluated to true?

Table 11 - ToBoolean Conversions

Undefined: false
Null: false
Boolean: The result equals the input argument (no conversion).
Number: The result is false if the argument is +0, -0, or NaN; otherwise the result is true.
String: The result is false if the argument is the empty String (its length is zero); otherwise the result is true.
Object: true

the answer should be yes. But then i'm wondering why:

alert(" " == false);

returns true. {same result with e.g "\t\n\r"}

I don't have any particular use case, I'm just wondering it.

See my answer here: http://stackoverflow.com/questions/10376179/n-t-r-0-is-true/10377034. `false` is converted to `0`. — Felix Kling, Jan 07 '14 at 18:41
Ok, i voted to close question because of duplicate... ;) Will accept first answer. Thx to you all. — A. Wolff, Jan 07 '14 at 18:54

score 1 · Accepted Answer · answered Jan 07 '14 at 18:42

It's because JavaScript describes whitespace only strings as having a toNumber conversion to 0.

You can test this using the unary + operator:

console.log(+"\t\n\r"); // 0

Because the == isn't so simple as a toBoolean conversion, you don't get the same result as you would if you did !!"\t\n\r", where the toBoolean conversion considers all non-empty strings to be true.

When the == is used with operands of different types, the algorithm usually coerces the types down until they're both numbers. That's why the toNumber conversion is relevant here.

@A.Wolff: Yes, that's an example of a whitespace-only string. Those are escape sequences that are just part of the literal syntax. The resulting string will only have whitespace. :-) — cookie monster, Jan 07 '14 at 18:45

score 1 · Answer 2 · answered Jan 07 '14 at 18:43

1

A StringNumericLiteral that is empty or contains only white space is converted to +0.

See page 44 in the PDF you linked

answered Jan 07 '14 at 18:43

Mirco

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Damn, thx! Page 44, noted – A. Wolff Jan 07 '14 at 18:43
http://stackoverflow.com/questions/4216362/javascript-white-space-string-boolean-conversion for a more detailed answer – Mirco Jan 07 '14 at 18:44
I can only accept one answer but guess you know that :( – A. Wolff Jan 07 '14 at 18:55
And ya, i stopped page 43, shame on me – A. Wolff Jan 07 '14 at 18:56

string to boolean conversion behaviour

2 Answers2