I was looking at the first table on /, and wanted to understand why, for example, 0 == [] and 0 != {}. I'm assuming it's because Number([]) == 0 and Number({}) == NaN. However, that part seems arbitrary. Why is an empty list 0 and empty object a NaN?

I was looking at the first table on http://zero.milosz.ca/, and wanted to understand why, for example, 0 == [] and 0 != {}. I'm assuming it's because Number([]) == 0 and Number({}) == NaN. However, that part seems arbitrary. Why is an empty list 0 and empty object a NaN?

• javascript

• 3 Arrays are weird like that. Like [[[[[[[[123]]]]]]]] == 123. – Niet the Dark Absol Commented Jun 21, 2012 at 18:42
• @Kolink ...but what causes this to work? (What rule of ==, not found in ===, is being applied?) – user166390 Commented Jun 21, 2012 at 18:44
• 4 @Kolink: You call that weird? (![]+[])[+[]]+(![]+[])[+!+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]] === 'fail' – ThiefMaster Commented Jun 21, 2012 at 18:48
• 1 @pst: The strict comparison returns false if the operands are not of the same type (as you probably know). But == will convert both operands to numbers in this case. – Felix Kling Commented Jun 21, 2012 at 18:56
• 1 Why does the title refer to === but the body use ==? Which one(s) is the question actually asking about? – Redwood Commented Jun 22, 2012 at 18:24

3 Answers 3

Using Number(some_object) will use the string representation of the given object. For your examples the string representations are:

js> ({}).toString();
[object Object]
js> [].toString();

js>

The string '[object Object]' cannot be converted to a number but the empty string '' can.

To elaborate a bit on ThiefMaster's answer, I've taken a look into ECMAScript's specifications:

When converting a string into a number, a grammar is used for the conversion. In particular, the mathematical value of StringNumericLiteral ::: [empty] is defined as 0. In fact, it's 0 for any whitespace.

When one value is an object ([],{}) and the other is a number or string, operator == converts the object to a primitive value (a number in this case) using the built-in conversion methods which all objects in Javascript inherit: toString() and valueOf().

For generic objects like {}, valueOf is used, and by default it returns the object itself, which is != 0.

For built-in arrays, toString is used. This method applied to an array returns a string containing all the elements joined by commas. For the empty array, it returns an empty string, ''.

Then the interpreter applies valueOf to that string; the return value of this method for an empty string is 0, so [] == 0.