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simbig wants to merge 2 commits into from
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Format decimal #50

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586b6bb
feat: add formatDecimal function for flexible decimal formatting
Sep 11, 2025
514c219
refactor: Apply eslint fix
simbig Sep 11, 2025
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32 changes: 31 additions & 1 deletion src/number.test.ts
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
import { firstDecimalDigit, round } from './number';
import { firstDecimalDigit, formatDecimal, round } from './number';

describe('round', () => {
it.each([
Expand Down Expand Up @@ -26,3 +26,33 @@ describe('firstDecimalDigit', () => {
expect(firstDecimalDigit(number)).toEqual(expectedResult);
});
});

describe('formatDecimal', () => {
it.each([
[
1.2345,
{ maxDecimals: 2, minDecimals: 1, trimTrailingZeros: true },
'1.23',
],
[
1.2,
{ maxDecimals: 3, minDecimals: 3, trimTrailingZeros: false },
'1.200',
],
[
1.2345,
{ maxDecimals: 4, minDecimals: 2, trimTrailingZeros: false },
'1.2345',
],
[1.2, { maxDecimals: 4, minDecimals: 2, trimTrailingZeros: true }, '1.2'],
[1, { maxDecimals: 2, minDecimals: 2, trimTrailingZeros: false }, '1.00'],
[
-1.234,
{ maxDecimals: 2, minDecimals: 0, trimTrailingZeros: true },
'-1.23',
],
[0, { maxDecimals: 3, minDecimals: 0, trimTrailingZeros: true }, '0'],
])('formatDecimal(%p, %p) → %p', (value, options, expected) => {
expect(formatDecimal(value, options)).toEqual(expected);
});
});
22 changes: 22 additions & 0 deletions src/number.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -22,3 +22,25 @@ export function firstDecimalDigit(number: number): number {

return Number(regExpMatchArray[1]);
}

export function formatDecimal(
value: number,
options: {
maxDecimals: number;
minDecimals: number;
trimTrailingZeros: boolean;
},
): string {
const { maxDecimals, minDecimals, trimTrailingZeros } = options;

const factor = 10 ** maxDecimals;
const rounded = Math.round(value * factor) / factor;
Comment on lines +36 to +37
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Copilot AI Sep 11, 2025

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This rounding logic duplicates the existing round function. Consider using round(value, maxDecimals) instead to avoid code duplication and maintain consistency.

Suggested change
const factor = 10 ** maxDecimals;
const rounded = Math.round(value * factor) / factor;
const rounded = round(value, maxDecimals);

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let str = rounded.toFixed(Math.max(minDecimals, maxDecimals));

if (trimTrailingZeros) {
str = str.replace(/(\.[0-9]*?)0+$/, '$1').replace(/\.$/, '');
Comment on lines +39 to +42
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Copilot AI Sep 11, 2025

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The logic for toFixed should use maxDecimals to ensure proper rounding, then handle minDecimals separately. Currently, when minDecimals > maxDecimals, this will display more decimals than the rounded value supports, potentially showing incorrect precision.

Suggested change
let str = rounded.toFixed(Math.max(minDecimals, maxDecimals));
if (trimTrailingZeros) {
str = str.replace(/(\.[0-9]*?)0+$/, '$1').replace(/\.$/, '');
// Always round to maxDecimals
let str = rounded.toFixed(maxDecimals);
if (trimTrailingZeros) {
// Remove trailing zeros, but ensure at least minDecimals remain
const [intPart, decPart = ''] = str.split('.');
let trimmedDec = decPart.replace(/0+$/, '');
// Pad with zeros if needed to reach minDecimals, but never more than maxDecimals
if (trimmedDec.length < minDecimals) {
trimmedDec = trimmedDec.padEnd(minDecimals, '0');
}
// If there are decimals, join; else, just intPart
str = trimmedDec.length > 0 ? `${intPart}.${trimmedDec}` : intPart;
} else if (minDecimals < maxDecimals) {
// If not trimming, pad with zeros to minDecimals if needed
const [intPart, decPart = ''] = str.split('.');
let paddedDec = decPart;
if (paddedDec.length < minDecimals) {
paddedDec = paddedDec.padEnd(minDecimals, '0');
}
str = paddedDec.length > 0 ? `${intPart}.${paddedDec}` : intPart;

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}

return str;
}