Tobie Langel

Comments

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  splintor says:

  The Javascript standard explains details in section 11.9.3 how the == operator is calculated when the two operands have different type.

  When both operands are String, Number or Boolean, their type is converted to Number, and then the comparison takes place. When both operands are null or undefined, the result of the comparison is true. However, one when of the operands is null or undefined, and the other is String, Number or Boolean, the result of the comparison is false.

  As explained in section 12.5, in the if statement evaluation uses the ToBoolean operator, which is explained in section 9.2 Where it says that 0 and '' are converted to true.

  Note that section 11.12 says that the conditional operator (?:) also uses the ToBoolean operator.

  I couldn’t find in the standard any explanation to this semantic difference between the ToBoolean operator and the == operator when null/undefined are involved.

  Sun, June 17 at 03:59 AM

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  rod says:

  I think trying… true == undefined and… true == null will go a way to explain it.

  Sun, June 17 at 08:30 AM

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  Tobie Langel says:

  Splintor: Thanks for the detailed explanation. Would you say it is like that because of a technical constraint ?

  Rod:

  true == undefined;
  // -> false
  true == null;
  // -> false

  Sun, June 17 at 12:56 PM

• 

  splintor says:

  I would say so, but I don’t see any such technical constraint from a brief look. However,I am dead tired…

  I don’t see why the equation operator couldn’t be defined that when one of the operands is null or undefined, the other operand would need to be converted to Boolean, and the operator would return true if that conversion returned false.

  Sun, June 17 at 14:22 PM

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  Adi Azar says:

  Thats quite interesting. I suffered from many javascript bugs similar to these in my life :)

  Sun, July 08 at 21:53 PM

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  Kevinin says:

  Just out of curiosity, I’ve tried the following:

  >>> new Boolean(0)
  false
  >>> new Boolean('')
  false
  >>> new Boolean(undefined)
  false
  >>> new Boolean(null)
  false
  

  And for the lack of optimism ;o)

  >>> new Boolean('false')
  true
  

  Sat, July 14 at 10:45 AM

• 

  David Brockman says:

  Doesn’t “if” behave more like ”!!...” than “false == ...”?

  
  NaN == NaN   // false
  false == NaN // false
  !!NaN        // false
  if (NaN) {}  // false
  

  Mon, July 16 at 06:46 AM

• 

  nexus says:

  Just because something evaluates to true/false in the context of a boolean expression doesn’t mean that that thing is therefore equivalent to true/false.

  Fri, July 20 at 19:14 PM

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  nexus says:

  As an example of what I mean, and continuing what David did:

  (!!undefined) === false;
  (!!null) === false;
  (!!NaN) === false;
  (!!false) === false;
  (!!0) === false; 
  (!!"") === false;
  

  (undefined == false) === false;
  (null == false) === false;
  (NaN == false) === false;
  (false == false) === true;
  (0 == false) === true;
  ("" == false) === true;

  Fri, July 20 at 19:20 PM

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  Tobie Langel says:

  Just because something evaluates to true/false in the context of a boolean expression doesn’t mean that that thing is therefore equivalent to true/false.

  Agreed, but in my opinion it’s still a violation of the principle of least surprise, especially considering 0 and '' are seen as equivalent to false.

  Sun, July 22 at 01:51 AM

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  Starlyght says:

  The operator can seem a bit weird at first, but it’s actually quite handy the way it is: if both sides of the operator are of the same type it checks if the arguments are indeed the same, else if the arguments are of a different type it checks if they both amount to the same truthy or falsy value. Use the = operator to do away with the typecheck and just check if the arguments are equal. I would recommend reading some of Douglas Crockford’s articles (http://javascript.crockford.com/), they are definitely worth the time.

  Mon, August 06 at 06:01 AM

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  Marty says:

  “especially considering 0 and ’’ are seen as equivalent to false“

  Not to mention ’ ’ (that’s a space). Also a false value.

  Mon, August 06 at 12:49 PM

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  Tobie Langel says:

  Starlyght: I think you are partially mixing up the === and == operators. === doesn’t check for equality, but for identity.

  Anyway, the weird issue here is that undefined’s truthy or falsy value, as you call it, is clearly false, and yet, false == undefined doesn’t evaluate to true.

  Marty: No it doesn’t. Any string but the empty string evaluates to true.

  Tue, August 07 at 14:10 PM

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  Jerod Venema says:

  I’m not sure about the evaulation of “undefined”, but the evaluation of “null” makes perfect sense to me. In the database world, null isn’t equal to ANYTHING else, even other nulls. They’ve slackened up on that definition a little in javascript, so null can be equal to null (and also, null == undefined), but the basic principal is still true. Null is neither true NOR false, it’s a non-state.

  Thu, August 09 at 08:18 AM

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  Chris Heilmann says:

  A the truthy/falsy fun with JavaScript.Mike has done quite a nice writeup after one of the Douglas Crockford talks in the office: http://www.isolani.co.uk/blog/javascript/TruthyFalsyAndTypeCasting

  Thu, August 09 at 15:12 PM

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  Mark Mailer says:

  I couldn’t find in the Javascript standard(explaining how ”==” operator is calculated when the two operands have different type) any explanation to this semantic difference between the Boolean and == operator whith null/undefined.

  Mon, September 03 at 01:52 AM

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  motoryzacja says:

  That is interesting :) I’m programing in php at my work so I does’nt have to make declaration of variables. But I remember a few problems when I try to compare variable === variable. It was some freaky results on my script. Anyway thanks for this article !

  Thu, September 13 at 22:26 PM

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