Javascript: string representation of numbers

Question

How does javascript convert numbers to strings? I expect it to round the number to some precision but it doesn't look like this is the case. I did the following tests:

> 0.1 + 0.2
0.30000000000000004
> (0.1 + 0.2).toFixed(20)
'0.30000000000000004441'
> 0.2
0.2
> (0.2).toFixed(20)
'0.20000000000000001110'

This is the behavior in Safari 6.1.1, Firefox 25.0.1 and node.js 0.10.21.

It looks like javascript displays the 17th digit after the decimal point for (0.1 + 0.2) but hides it for 0.2 (and so the number is rounded to 0.2).

How exactly does number to string conversion work in javascript?

Answer 1

From the question's author:

I found the answer in the ECMA script specification: http://www.ecma-international.org/ecma-262/5.1/#sec-9.8.1

When printing a number, javascript calls toString(). The specification of toString() explains how javascript decides what to print. This note below

The least significant digit of s is not always uniquely determined by the requirements listed in step 5.

as well as the one here: http://www.ecma-international.org/ecma-262/5.1/#sec-15.7.4.5

The output of toFixed may be more precise than toString for some values because toString only prints enough significant digits to distinguish the number from adjacent number values.

explain the basic idea behind the behavior of toString().