Why eval("475957E-8905") == "475957E-8905" is true?

Question

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" ?

Answer 1

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.

Answer 2

Javascript has to convert your string, "475957E-8905", to a number in order to compare them. When it does that, it converts "475957E-8905" to 0 as well. So, 0 == 0;

As you can see:

"475957E-8905" == 0

Is true. Basically you eval statement "475957E-8905" into a number, and then the other "475957E-8905" is being converted to a number for comparison. In the end, the exact same conversion process has happened to both of them and they are both 0.