Why Does {} == False Evaluate To False While [] == False Evaluates To True?

Why does {} == false evaluate to false while [] == false evaluates to true in javascript?

Solution 1:

This is the type conversion that takes place according to the Abstract Equality Comparison Algorithm:

{} == false// step 7 {} == ToNumber(false)
{} == 0// step 9 ToPrimitve({}) == 0"[object Object]" == 0// step 5 ToNumber("[object Object]") == 0
NaN == 0// step 1.c.i

[] == false// step 7 [] == ToNumber(false)
[] == 0// step 9 ToPrimitve([]) == 0"" == 0// step 5 ToNumber("") == 00 == 0// step 1.c.iii

References: ToNumber, ToPrimitive

And because of this, prefer to use strict comparison.

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Some examples how ToPrimitive converts objects to primitives during comparison. By default, the valueOf method of the object will be called, and then toString if valueOf doesn't return a primitive value. For Date objects it will call toString by default.

var obj = {};
obj.valueOf();        // Object { } // the object itself
obj.toString();       // "[object Object]"


obj.valueOf = function() { return123; };
obj == 123; // true

obj.toString = function() { return'foo bar'; };
obj == 123; // false
obj == 'foo bar'; // true// Date objectvar date = newDate();
date.valueOf();        // 1421430720379
date.toString();       // "Fri Jan 16 2015 09:52:00 GMT-0800 (PST)"

date == 1421430720379// false
date == "Fri Jan 16 2015 09:52:00 GMT-0800 (PST)"// true

date.toString = function() { return'foo'; };
date == 'foo'; // true