Jazib - 1 year ago 112
Javascript Question

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

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?

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
``````
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