JS: Format Number

By Xah Lee. Date: . Last updated: .

JavaScript does not have a “format” or “printf” function. The following code are solutions.

Show Only N Decimal Places

Use toFixed.

〔see Number.prototype.toFixed〕

Show Only N Significant Digits

Use toPrecision, but result may be in exponential notation.

〔see Number.prototype.toPrecision〕

Show in Exponential Notation

Use toExponential

Example output: "3.12e+2"

〔see Number.prototype.toExponential〕

Format Number with Metric Prefix

const xah_format_number = ((n, m = 1) => {
  /*
format number with metric prefix, e.g. 1.2 k
n is integer. The number to be converted
m is integer. The number of decimal places to show. Default to 1.
returns a string, with possibly one of k M G T ... suffix.

http://xahlee.info/js/js_format_number.html
version 2019-04-15
 */
  const prefix = [
    "",
    " k",
    " M",
    " G",
    " T",
    " P",
    " E",
    " Z",
    " Y",
    " * 10^27",
    " * 10^30",
    " * 10^33",
  ]; // should be enough. Number.MAX_VALUE is about 10^308
  let ii = 0;
  while ((n = n / 1000) >= 1) ii++;
  return (n * 1000).toFixed(m) + prefix[ii];
});

/* test--------------------------------------------------- */

console.log(
  xah_format_number(111, 1) === "111.0",
  xah_format_number(111222, 1) === "111.2 k",
  xah_format_number(111222333, 1) === "111.2 M",
  xah_format_number(111222333444, 1) === "111.2 G",
  xah_format_number(111222333444) === "111.2 G",
  xah_format_number(111222333444, 3) === "111.222 G",
  xah_format_number(111222333444, 0) === "111 G",
);

JavaScript, Number