Question

jsbin首先警告我,x !=不是好的,我将改为x !=。

为什么?

Answer 1

var x = false;
console.log(x !==   ); //true
console.log(x !=   ); //false

In other words, false (and other falsy values, like 0) will coerce to an empty string. The !== and === operators (strict equality operators) ensure that the things being compared are of the same type.

To expand upon why this is the case, you need to head to the spec (linked to by T.J. Crowder in the comments). The section on the "Abstract Equality Comparison Algorithm" tells us the following:

If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.

>>>> 告诉我们:

The result is 1 if the argument is true. The result is +0 if the argument is false.

在上述例子中,论点是false,因此我们现在将+0 !=加以比较。在将数字与地体进行比较时,遵循以下规则:

If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).

Calling ToNumber on an empty string results in +0, just as it did for false:

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

现在我们正在比较<代码>+0 ! = +0,这样我们就进入“x and y are of same”部分,这告诉我们:

如果X是同数值一样的,回报是真实的。

因此,<代码>+0等于+0,而且由于我们正在使用<代码>!,该代码返回<>false<>/code>。

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