Thousands Separator

A thousands separator groups the digits of a number into runs of three — turning 1234567 into 1,234,567 — so a person can read the magnitude at a glance instead of counting zeros. You will write thousandsSeparator(value, options), which returns that grouped string. It has to group the integer part from the right, leave the decimals alone, keep a negative sign in front, and honor a custom separator and decimal mark. Browsers ship this in Intl.NumberFormat, but here you build the grouping by hand.

Signature

thousandsSeparator(
  value: number | string,
  options?: { separator?: string; decimal?: string }
): string
// separator defaults to ",", decimal defaults to "."

Examples

thousandsSeparator(1234567.89);  // '1,234,567.89'
thousandsSeparator(-1000);       // '-1,000'
thousandsSeparator(999);         // '999'
thousandsSeparator(1000000);     // '1,000,000'

thousandsSeparator(1234567, { separator: ' ' });                  // '1 234 567'
thousandsSeparator(1234567.89, { separator: '.', decimal: ',' }); // '1.234.567,89'
thousandsSeparator('1234567.89');                                 // '1,234,567.89'

Notes

• Group from the right — the integer part splits into runs of three starting at the ones place, so the leftmost group may hold 1, 2, or 3 digits.
• The fraction is never grouped — digits after the decimal point stay as a single run; only the integer part receives separators.
• Sign stays in front — a negative value keeps its leading - before the first digit.
• Number or string — value may be a JS number or a numeric string. A string lets a caller preserve exact decimals, such as a trailing zero that a number would drop.
• Build it yourself — implement the grouping directly; do not delegate to Intl.NumberFormat or Number.prototype.toLocaleString.
• Assume a normal magnitude — you can ignore values so large that JS prints them in exponential form (like 1e21), and you do not need to validate non-numeric junk.