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

偶尔善良 提交于 2019-11-28 01:53:54

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("") == 0
0 == 0                     // step 1.c.iii

References: ToNumber, ToPrimitive

And because of this, prefer to use strict comparison.


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() { return 123; };
obj == 123; // true

obj.toString = function() { return 'foo bar'; };
obj == 123; // false
obj == 'foo bar'; // true

// Date object

var date = new Date();
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
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