Truncate long strings, but don't break in middle of a short word

truncate.test.ts

import truncate from "./truncate";

describe("Truncate string", () => {
  test("empty string returns self", () => expect(truncate("")).toBe(""));

  test("short string returns self", () => {
    expect(truncate("word", 8)).toBe("word");
    expect(truncate("too long", 8)).toBe("too long");
  });

  test("long string truncated at specified length", () => {
    expect(truncate("ETOOMANYCHARS", 6)).toBe("ETOOM…");
    expect(truncate("ETOOMANYCHARS", 12)).toBe("ETOOMANYCHA…");
  });

  test("short word truncated by dropping", () => {
    expect(truncate("one two three", 6)).toBe("one …");
    expect(truncate("one two three", 7)).toBe("one …");
    expect(truncate("one two three", 8)).toBe("one two…");
    expect(truncate("one two three", 9)).toBe("one two …");
  });

  test("long words truncated with ellipsis", () => {
    expect(truncate("one toolong", 9)).toBe("one …");
    expect(truncate("one toolong", 10)).toBe("one toolo…");
  });
});

truncate.ts

const minWordLengthToBreak = 5;
const dropShortWord = RegExp(`\\s\\S{1,${minWordLengthToBreak - 1}}$`);

export default function truncate(text: string, maxChars = 280): string {
  if (text.length <= maxChars || maxChars < minWordLengthToBreak) return text;
  const truncated = text.slice(0, maxChars - 1);
  const followedByWhitespace = /\s/.test(text[maxChars - 1]!);
  if (followedByWhitespace) return `${truncated}…`;
  return `${truncated.replace(dropShortWord, " ")}…`;
}