Add formatting implementation, with a unittest for thousands separators

lib/format.ts

@@ -0,0 +1,64 @@
+import {toFixed} from './index';
+
+interface FormatOptions {
+  minDecimals: number;
+  maxDecimals: number;
+  percent?: boolean;      // Format as percentage
+  grouping?: string;      // String for thousand separators
+  accounting?: boolean;   // Use parentheses for negative numbers
+  exponential?: boolean;  // Use exponential notation
+  currency?: string;      // Symbol for currency prefix
+}
+
+type FormatFunc = (num: number) => string;
+
+// TODO need separate tests for addGrouping, and core createFormatFunc; should find assorted test
+// cases for it.
+// Add benchmarks comparing to Intl and numeraljs.
+export function createFormatFunc(options: FormatOptions): FormatFunc {
+  const {minDecimals, maxDecimals, grouping, currency} = options;
+  if (options.exponential) {
+    return (num: number) => num.toExponential(maxDecimals);
+  }
+  let func: FormatFunc;
+  if (grouping) {
+    func = (num: number) => addGrouping(toFixed(num, minDecimals, maxDecimals), grouping);
+  } else {
+    func = (num: number) => toFixed(num, minDecimals, maxDecimals);
+  }
+  if (options.accounting) {
+    const func1 = func;
+    func = (num: number) => num < 0 ? `(${func1(-num)})` : func1(num);
+  }
+  if (options.percent) {
+    const func1 = func;
+    func = (num: number) => func1(num * 100) + '%';
+  } else if (options.currency) {
+    const func1 = func;
+    if (options.accounting) {
+      func = (num: number) => currency + func1(num);
+    } else {
+      func = (num: number) => num < 0 ? '-' + currency + func1(-num) : currency + func1(num);
+    }
+  }
+  return func;
+}
+
+/**
+ * Add thousands separators to the input number, which should be in the format /-?\d*(.\d*)?/
+ * (e.g. as returned by toFixed).
+ */
+export function addGrouping(input: string, sep: string = ','): string {
+  let decimalDot = input.indexOf('.');
+  let start = input[0] === '-' ? 1 : 0;
+  let end = decimalDot < 0 ? input.length : decimalDot;
+  let digits = end - start;
+  let firstGroup = (digits + 2) % 3 + 1;    // Mod 3, but returning a value between 1 and 3.
+  let p = start + firstGroup;
+  let res = input.slice(0, p);
+  for ( ; p < end; p += 3) {
+    res += sep + input.slice(p, p + 3);
+  }
+  res += input.slice(p);
+  return res;
+}

test/test-format.ts

@@ -0,0 +1,50 @@
+import {assert} from 'chai';
+import {toFixed} from '../lib/index';
+import {addGrouping} from '../lib/format';
+
+describe('format', () => {
+  describe('addGrouping', function() {
+    it('should add thousand separators correctly', function() {
+      assert.equal(addGrouping(''), '');
+      assert.equal(addGrouping('0'), '0');
+      assert.equal(addGrouping('10'), '10');
+      assert.equal(addGrouping('100'), '100');
+      assert.equal(addGrouping('1000'), '1,000');
+      assert.equal(addGrouping('1234'), '1,234');
+      assert.equal(addGrouping('12345'), '12,345');
+      assert.equal(addGrouping('123456'), '123,456');
+      assert.equal(addGrouping('1234567'), '1,234,567');
+
+      // Same as above, with a minus sign.
+      assert.equal(addGrouping('-0'), '-0');
+      assert.equal(addGrouping('-10'), '-10');
+      assert.equal(addGrouping('-100'), '-100');
+      assert.equal(addGrouping('-1000'), '-1,000');
+      assert.equal(addGrouping('-1234'), '-1,234');
+      assert.equal(addGrouping('-12345'), '-12,345');
+      assert.equal(addGrouping('-123456'), '-123,456');
+      assert.equal(addGrouping('-1234567'), '-1,234,567');
+
+      // Same as above, with decimals, assorted minus signs.
+      assert.equal(addGrouping('-0.1234567'), '-0.1234567');
+      assert.equal(addGrouping('.1234567'), '.1234567');
+      assert.equal(addGrouping('-.1234567'), '-.1234567');
+      assert.equal(addGrouping('-10.1234567'), '-10.1234567');
+      assert.equal(addGrouping('100.1234567'), '100.1234567');
+      assert.equal(addGrouping('-1000.1234567'), '-1,000.1234567');
+      assert.equal(addGrouping('1234.1234567'), '1,234.1234567');
+      assert.equal(addGrouping('-12345.1234567'), '-12,345.1234567');
+      assert.equal(addGrouping('123456.1234567'), '123,456.1234567');
+      assert.equal(addGrouping('-1234567.1234567'), '-1,234,567.1234567');
+
+      // Very large numbers
+      assert.equal(addGrouping(toFixed(6.0015e20, 0, 20)), '600,150,000,000,000,000,000');
+    });
+
+    it('should support custom separators', function() {
+      assert.equal(addGrouping(toFixed(6.0015e20, 0, 20), "'"), "600'150'000'000'000'000'000");
+      assert.equal(addGrouping(toFixed(6.0015e20, 0, 20), " "), "600 150 000 000 000 000 000");
+      assert.equal(addGrouping('-1234567.1234567', ' '), '-1 234 567.1234567');
+    });
+  });
+});