Strange syntax of Number methods in JavaScript

Question

Take a look at the following code:

Number.prototype.isIn = function () {
    for (var i = 0, j = arguments.length; i < j; ++i) {
        if (parseInt(this, 10) === arguments[i]) {
            return true;
        }
    }
    return false;
};

var x = 2;
console.log(x.isIn(1,2,3,4,5)); // <= 'true'
console.log(2.isIn(1,2,3,4,5)); // <= Error: 'missing ) after argument list'

Why is it that when it's a variable, the code works correctly yet when it is a number literal, it fails ?

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And also, strangely enough, why does the following line work?

console.log((2).isIn(1,2,3,4,5)); // <= 'true'

In the above line, I basically enclosed the literal in parenthesis.

Answer 1

It's a syntax error because you are representing a number. Strings can work that way, but not numbers, because a period immediately following a number symbolizes a decimal value. The character after the . is causing the error.

Answer 2

Most of the answers already stated that a dot after a numeric literal is considered part of this number as a fraction separator. But if you still want to use the dot as an operator then the quick and easy fix would be leaving an empty space between the number and the space.

2 .isIn(1,2,3,4,5) // <- notice the space between 2 and .