+/-一元运算符计算空字符结果为0分析

  JavaScript一元运算符对空字符计算时为什么得到0,代码如下

<script>
alert(+'')//0
alert(+[])//0
</script>

  这是因为对空字符使用一元运算符+/-时,空字符会被强制转为0或者将字符串传入Number构造函数中,而Number('')返回0。

<script>
alert(Number(''))//0
</script>

  +[]为什么也是0,是应为+和对象运算,会调用对象的toString()方法,空数组即[].toString()返回的是空字符串,所以+[]等价于+""。

  下面是来自stackoverflow的详细解答。

 

来源:http://stackoverflow.com/questions/3306453/why-an-empty-array-type-converts-to-zero

In a brief, there are two key points:

For example, we can use an object that defines a toString method, and returns an empty string to have an equivalent result:

var obj = { toString: function () { return "";} };
+obj; //  0
Number(obj); // 0

This behavior is completely standard.

Now the long answer:

Both, the unary plus operator and the Number constructor called as a function internally use the ToNumber abstract operation.

ToNumber will use two more internal operations, ToPrimitive and [[DefaultValue]].

When the ToNumber operation is applied to an Object, such the empty array in your example, it calls the ToPrimitive operation, to get a representative primitive value and call ToNumber again using that value [1].

The ToPrimitive operation receive two arguments, a Value (which is your array object), and a hint type, which in this case is "Number" since we want to make numeric conversion.

ToPrimitive calls the [[DefaultValue]] internal method, also with a "Number" hint type.

Now, since the hint type we are using "Number", the [[DefaultValue]] internal method now will try to invoke first the valueOf method on the object.

Array objects don't have a specific valueOf method, the method is the one inherited from Object.prototype.valueOf, and this method simply returns a reference to the object itself.

Since the valueOf method didn't result in a primitive value, now the toString method is invoked, and it produces an empty string (which is a primitive value), then the ToNumber operation will try to do String-Number conversion and it finally end up with 0 [1].

But now you might wonder, why an empty string coerces to zero?

+""; // 0

There is a complete grammar that is used when the ToNumber internal operation is applied to a String type, the StringNumericLiteral production.

It has some differences between a NumericLiteral, and one of those differences is that:

A StringNumericLiteral that is empty or contains only white space is converted to +0.

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