parseInt() parses number literals with exponent incorrectly

Question

I have just observed that the parseInt function doesn't take care about the decimals in case of integers (numbers containing the e character).

Let's take an example: -3.67394039744206e-15

> parseInt(-3.67394039744206e-15)
-3
> -3.67394039744206e-15.toFixed(19)
-3.6739e-15
> -3.67394039744206e-15.toFixed(2)
-0
> Math.round(-3.67394039744206e-15)
0

I expected that the parseInt will also return 0. What's going on at lower level? Why does parseInt return 3 in this case (some snippets from the source code would be appreciated)?

In this example I'm using node v0.12.1, but I expect same to happen in browser and other JavaScript engines.

Answer 1

I think the reason is parseInt converts the passed value to string by calling ToString which will return "-3.67394039744206e-15", then parses it so it will consider -3 and will return it.

The mdn documentation

The parseInt function converts its first argument to a string, parses it, and returns an integer or NaN

Answer 2

parseInt(-3.67394039744206e-15) === -3

The parseInt function expects a string as the first argument. JavaScript will call toString method behind the scene if the argument is not a string. So the expression is evaluated as follows:

(-3.67394039744206e-15).toString()
// "-3.67394039744206e-15"
parseInt("-3.67394039744206e-15")
// -3

-3.67394039744206e-15.toFixed(19) === -3.6739e-15

This expression is parsed as:

• Unary - operator
• The number literal 3.67394039744206e-15
• .toFixed() -- property accessor, property name and function invocation

The way number literals are parsed is described here. Interestingly, +/- are not part of the number literal. So we have:

// property accessor has higher precedence than unary - operator
3.67394039744206e-15.toFixed(19)
// "0.0000000000000036739"
-"0.0000000000000036739"
// -3.6739e-15

Likewise for -3.67394039744206e-15.toFixed(2):

3.67394039744206e-15.toFixed(2)
// "0.00"
-"0.00"
// -0