javascript - + Javascript 中的运算符

Question

所以我注意到

的结果

(new Date())

是一个日期对象；在这种情况下

Date {Thu Dec 04 2014 22:43:07 GMT+0200 (SAST)}

但是如果我输入

+(new Date())

我得到一个整数值；

1417725787989

这是怎么做到的？

我有一个名为“Duration”的函数，当它像这样使用时:

new Duration(352510921)

返回一个看起来像这样的实例:

{ days:5, hours:3, mins:55, secs:10, ms:921 }    

那么如何使用 + 运算符获取 Duration 实例的 int 值呢？

var dur = new Duration(352510921);
console.log(+dur) // prints int value 352510921

Answer 1

一元 + 运算符将实例转换为 Number，就像调用 Number() 函数将变量转换为一个数字。

如果您想覆盖特定实例的转换方式，您需要覆盖该实例的 valueOf 属性:

var a = {
    valueOf: function () {
        return 5;
    }
};
console.log(a); //Object { valueOf: function () {...} }
console.log(+a); //5

来自ES5 standard :

11.4.6 Unary + Operator # Ⓣ Ⓡ Ⓖ

The unary + operator converts its operand to Number type.

The production UnaryExpression : + UnaryExpression is evaluated as follows:

1. Let expr be the result of evaluating UnaryExpression.
2. Return ToNumber(GetValue(expr)).

对象上的

ToNumber 将导致 ToPrimitive与提示号码。 ToPrimitive 将调用 [[DefaultValue]]内部方法指出:

When the [[DefaultValue]] internal method of O is called with hint Number, the following steps are taken:

1. Let valueOf be the result of calling the [[Get]] internal method of object O with argument "valueOf".
2. If IsCallable(valueOf) is true then,
   1. Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this value and an empty argument list.
   2. If val is a primitive value, return val.
3. Let toString be the result of calling the [[Get]] internal method of object O with argument "toString".
4. If IsCallable(toString) is true then,
   1. Let str be the result of calling the [[Call]] internal method of toString, with O as the this value and an empty argument list.
   2. If str is a primitive value, return str.
5. Throw a TypeError exception.