为什么eval（“475957E

为什么eval（“475957E-8905”）==“475957E-8905”是真的？(Why eval(“475957E-8905”) == “475957E-8905” is true?)

我用nodeJs制作了一个程序，可以生成如下代码

eval("XXXXXX") == "XXXXXX"

它工作得很好，但是他立刻给了我这个：

eval("475957E-8905") == "475957E-8905"

我用Firebug对它进行了测试，结果是true 。但我不明白为什么。

当然， eval("475957E-8905")返回0但为什么0 == "475957E-8905" ？

I made a program with nodeJs which generate code such as

eval("XXXXXX") == "XXXXXX"

It's working pretty well, but at a moment he gave me this :

eval("475957E-8905") == "475957E-8905"

I tested it with Firebug, and the result is true .But I don't really understand why.

Of course, eval("475957E-8905") return 0 but why 0 == "475957E-8905" ?

最满意答案

这个难题有两个部分：浮点数和使用==类型不敏感比较。

首先， 475957E-8905评估为浮点数475957 * 10 ^ -8905 ，这是令人难以置信的小; 在浮点方面，由于javascript的精度限制，它与0相同。 所以， eval("475957E-8905")返回0 。

现在，为拼图的第二部分。

==表示类型不必匹配，所以nodejs（就像任​​何JavaScript引擎）试图转换其中的一个，以便它们可以比较它们。

由于eval("475957E-8905")返回0 ，因此它会尝试将"475957E-8905"转换为整数。 正如我们所见，那也是0 。 因此，比较结果为0 == 0 ，这是正确的。

请注意，如果您执行eval("3") == "3"或eval("3") == 3 ，则会发生同样的情况 - 在每种情况下，都将字符串转换为数字并进行比较。

避免这个问题

你可以像这样强制类型敏感的比较：

eval("475957E-8905") === "475957E-8905"

它返回false，因为===告诉javascript引擎只有在类型和值都匹配时才返回true。

There are two pieces to this puzzle: floating-point numbers and type-insensitive comparison using ==.

First, 475957E-8905 evaluates as the floating point number 475957 * 10 ^ -8905, which is incredibly small; in floating-point terms, it's the same as 0 due to the precision limitations of javascript. So, eval("475957E-8905") returns 0.

Now, for the second piece of the puzzle.

The == means that the types don't have to match, so nodejs (like any JavaScript engine) tries to convert one of them so it can compare them.

Since eval("475957E-8905") returned 0, it tries to convert "475957E-8905" to an integer as well. As we have seen, that is also 0. Thus, the comparison is 0 == 0, which is true.

Note that the same thing happens if you do eval("3") == "3" or eval("3") == 3 -- in each case, the strings are converted to numbers and compared.

Avoiding this problem

You can force a type-sensitive comparison like this:

eval("475957E-8905") === "475957E-8905"

which returns false, because the === tells the javascript engine to return true only if the types and the values both match.