format.js

import {format as isoformat} from "isoformat";
import {html} from "htl";

// Note: use formatAuto (or any other localized format) to present values to the
// user; stringify is only intended for machine values.
export function stringify(x) {
  return x == null ? "" : `${x}`;
}

export const formatLocaleAuto = localize(locale => {
  const formatNumber = formatLocaleNumber(locale);
  return value => value == null ? ""
    : typeof value === "number" ? formatNumber(value)
    : value instanceof Date ? formatDate(value)
    : `${value}`;
});

export const formatLocaleNumber = localize(locale => {
  return value => value === 0 ? "0" : value.toLocaleString(locale); // handle negative zero
});

export function formatObjects(format, circular = true) {
  return value => value === null ? gray("null")
  : value === undefined ? gray("undefined")
  : Number.isNaN(value) ? gray(NaN)
  : circular && Array.isArray(value) ? formatArray(value)
  : circular && isTypedArray(value) ? formatArray(value)
  : value instanceof Date ? formatDate(value)
  : circular && value instanceof Node ? value
  : circular && typeof value === "object" ? formatObject(value)
  : format(value);
}

function isTypedArray(_) {
  return _?.prototype?.__proto__?.constructor?.name == "TypedArray";
}

function formatObject(o) {
  const subformat = formatObjects(formatAuto, false);
  return `{${Object.entries(o).map(([key, value]) => `${key}: ${subformat(value)}`).join(", ")}}`;
}

function formatArray(value) {
  const subformat = formatObjects(formatAuto, false);
  const l = value.length;
  value = Array.from({length: Math.min(30, l)}, (_, i) => subformat(value[i]));
  return `[${value.join(", ")}${l > 30 ? "…" : "" }]`;
}

export const formatAuto = formatLocaleAuto();

export const formatNumber = formatLocaleNumber();

export function formatTrim(value) {
  const s = value.toString();
  const n = s.length;
  let i0 = -1, i1;
  out: for (let i = 1; i < n; ++i) {
    switch (s[i]) {
      case ".": i0 = i1 = i; break;
      case "0": if (i0 === 0) i0 = i; i1 = i; break;
      default: if (!+s[i]) break out; if (i0 > 0) i0 = 0; break;
    }
  }
  return i0 > 0 ? s.slice(0, i0) + s.slice(i1 + 1) : s;
}

export function formatDate(date) {
  return isoformat(date, "Invalid Date");
}

// Memoize the last-returned locale.
export function localize(f) {
  let key = localize, value;
  return (locale = "en") => locale === key ? value : (value = f(key = locale));
}

function gray(label) {
  return html`<span class=gray>${label}`;
}