Javascript - Most efficient way to round off float to nearest non-zero number

Question

I want to round off a float in Javascript, with (at least) 2 significant figures.

For example, with following desired results:

roundOff(0.000000001124)  // 0.0000000011 or 1.1e-9 (both acceptable)
roundOff(1.423421)  // 1.42
roundOff(0.00053245)  // 0.00053
roundOff(422.243224)  // 422.24
roundOff(422.0000000123)  // 422.00

I know I can use logarithm and round to do something similar as mentioned here, but it rounds off even larger numbers, like -123.0 -> -100.

Any clever Javascript based solution?

Answer 1

If I understand correctly, you want a function that returns two significant figures for numbers with an absolute value less than 1, or two decimal places for numbers with an absolute value greater than 1.

I still don't understand why you think 0.000000001124 should be rounded to 0.0000000012. Did you mean to write 0.0000000011 instead?

If so, this function should do what you want:

function roundOff(n) {
  return parseFloat(n.toExponential(Math.max(1,2+Math.log10(Math.abs(n)))));
}

Examples:

var a = new Array(0.000000001124, 1.423421, 0.00053245, 422.243224,
                  422.0000000123, 123, -123);
for (i=0; i<7; i++) console.log(roundOff(a[i]));

Output:

1.1e-9
1.42
0.00053
422.24
422
123
-123