为什么 {} == false 评估为 false 而 [] == false 评估为 true?
为什么 {} == false 评估为 false 而 [] == false 评估为 true在 JavaScript 中?
Why does {} == false evaluate to false while [] == false evaluates to true in javascript?
推荐答案
这是根据抽象等式比较算法:
{} == 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
参考:ToNumber，ToPrimitive
因此，更喜欢使用严格比较.
And because of this, prefer to use strict comparison.
ToPrimitive 如何在比较过程中将对象转换为基元 的一些示例.默认情况下，将调用对象的 valueOf 方法，如果 valueOf 没有返回原始值，则调用 toString.对于 Date 对象，它会默认调用 toString.
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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