Truncate middle

truncate_middle.js

export function truncateMiddle(text: string, truncationLength: number) {
  if (text.length <= truncationLength || truncationLength <= ELLIPSIS_WIDTH) {
    return text;
  }

  // ceil to favor an extra char or two before vs after the ellipsis
  const halfLength = Math.ceil((truncationLength - ELLIPSIS_WIDTH) / 2);
  const preEllipsisText = text.substr(0, halfLength).trim();
  const postEllipsisText = text
    .substr(
      text.length - (truncationLength - ELLIPSIS_WIDTH - preEllipsisText.length)
    )
    .trim();

  return `${preEllipsisText}...${postEllipsisText}`;
}

describe("#truncateMiddle", () => {
  it("does not modify a string that is too short to need truncation", () => {
    const text = "Never confuse movement with action.";

    expect(truncateMiddle(text, text.length + 1)).toEqual(text);
  });

  it("does not truncate below the width of the ellipsis", () => {
    const text = "Never confuse movement with action.";

    expect(truncateMiddle(text, 3)).toEqual(text);
  });

  it("overweights the first half of the string when truncation will not be balanced", () => {
    const text = "Never confuse movement with action.";

    expect(truncateMiddle(text, 4)).toEqual("N...");
    expect(truncateMiddle(text, 8)).toEqual("Nev...n.");
  });

  it("ellides from the middle of a string to an odd length", () => {
    const text =
      "The first principle is that you must not fool yourself and you are the easiest person to fool.";

    expect(truncateMiddle(text, 33)).toEqual(
      "The first princ...person to fool."
    );
  });

  it("ellides from the middle of a string to an even length", () => {
    const text =
      "The first principle is that you must not fool yourself and you are the easiest person to fool.";

    expect(truncateMiddle(text, 34)).toEqual(
      "The first princi...person to fool."
    );
  });

  it("omits trailing space from the text before the ellipsis mark", () => {
    const text = "Never confuse movement with action.";

    expect(truncateMiddle(text, 15)).toEqual("Never...action.");
  });

  it("may produce a string shorter than specified if a leading space is trimmed from the second half", () => {
    const text = "Never confuse movement with action.";

    expect(truncateMiddle(text, 29)).toEqual("Never confuse...with action.");
    expect(truncateMiddle(text, 29)).toHaveLength(28);
  });
});