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If I write the following code:

" \t\r\n" == 0

JavaScript will return true. However, if I manually convert both values to boolean, then:

!!" \t\r\n" // returns true
!!0 // returns false

That's mean JavaScript says that true == false in the first expression, but the opposite if I compare values converted to boolean, which doesn't make any sense to me.

Please note that I don't write !!"0" but !!0 instead - big difference.

Nisse Engström
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KBS
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  • http://stackoverflow.com/questions/359494/does-it-matter-which-equals-operator-vs-i-use-in-javascript-comparisons – BassT Dec 20 '15 at 14:27
  • this does not answer my question. I don't use === operator here – KBS Dec 20 '15 at 14:28
  • @BassT JavaScript should convert both values to boolean if I use == and this is not working as shown above – KBS Dec 20 '15 at 14:29
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    Possible duplicate of [why do both if('0'==false) and if('0') evaluate to true in Javascript?](http://stackoverflow.com/questions/16450750/why-do-both-if0-false-and-if0-evaluate-to-true-in-javascript) – JJJ Dec 20 '15 at 14:29
  • *"JavaScript should convert both values to boolean if I use =="* -- that's just not true. – JJJ Dec 20 '15 at 14:30
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    What makes you think javascript should convert both values to boolean ? – adeneo Dec 20 '15 at 14:30
  • @Juhana the answer provided by BassT says that "The == operator will compare for equality after doing any necessary type conversions." which means it convert both values to boolean – KBS Dec 20 '15 at 14:31
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    Noooo, it means it converts both values to ***the same type***, not boolean. – adeneo Dec 20 '15 at 14:32

2 Answers2

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The comparison 

" \t\r\n" == 0

is true because the specification for the language says it should be true.

Lets start with the algorithm for Abstract Equality Comparison (x == y)

If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.

See what it says there, if the first one is a string, which it is, and the second one is a number, which it is, convert the first one to a number based on the ToNumber() operation.

So what's the ToNumber() operation, here it is in the spec

ToNumber applied to Strings applies the following grammar to the input String.
If the grammar cannot interpret the String as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.

The entire chapter on how different strings are converted with ToNumber is pretty complicated, but a little further down in the spec is says

The MV of StringNumericLiteral ::: [empty] is 0.
The MV of StringNumericLiteral ::: StrWhiteSpace is 0.

(MV = Mathematical Value)

So any string that is empty, or just contain whitespace, is converted to 0.

Lets just try and coerce the string ourselves

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

So a string containing a whitespace, a tab, and a newline, is converted to 0.

So, converting the string " \t\r\n" with the internal ToNumber() operation gives the browser 0, so naturally 0 == 0 is true, and that's the answer.

adeneo
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0

First of all you're using equality and not strict equality to compare two values.

When using == (equality) values will be coerced. This won't happen when using === (strict equality).

" \t\r\n" == 0 // returns true, these values are 'falsy', which are coerced to false 

" \t\r\n" === 0 // returns false

See this question for reference.

Community
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danillouz
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