Question

    ​document.writeln(Math.floor(43.9));

​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​produces 43 in the browser.

​document.writeln(Math.floor(43.9999));​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

produces 43

 ​document.writeln(Math.floor(43.999999999999));​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​ 

again 43

However,

    document.writeln(Math.floor(43.99999999999999));

produces 44.

The magic number of 9’s after the decimal point seems to be 15*.

Why is this?

Furthermore, Does the Math.floor function accept the number as a number object, or a number value?

Answer 1

The IEEE 754 double-precision binary floating-point format (which is what JavaScript uses for its Number type) gives you a precision of 15 – 17 significant decimal digits.

This gives from 15 – 17 significant decimal digits precision. If a
decimal string with at most 15 significant decimal is converted to
IEEE 754 double precision and then converted back to the same number
of significant decimal, then the final string should match the
original; and if an IEEE 754 double precision is converted to a
decimal string with at least 17 significant decimal and then converted
back to double, then the final number must match the original [1].

Source: http://en.wikipedia.org/wiki/Double-precision_floating-point_format#IEEE_754_double-precision_binary_floating-point_format:_binary64