Function assertCharDiff

ts/codediff/__tests__/addcharacterdiffs_test.ts:169–199

(
    beforeText: string,
    beforeExpectation: string,
    afterText: string,
    afterExpectation: string,
  )

  });

  function assertCharDiff(
    beforeText: string,
    beforeExpectation: string,
    afterText: string,
    afterExpectation: string,
  ) {
    const codes = computeCharacterDiffs(beforeText, afterText);
    if (codes === null) {
      expect(codes).not.toBeNull();
      throw new Error();
    }
    // 'Declined to generate a diff when one was expected.');

    const [beforeCodes, afterCodes] = codes;

    const process = function (codes: CharacterDiff[], txt: string) {
      return codes
        .map(function (code) {
          let part = txt.substring(code[1], code[2]);
          if (code[0] != null) part = '[' + part + ']';
          return part;
        })
        .join('');
    };

    const beforeActual = process(beforeCodes, beforeText),
      afterActual = process(afterCodes, afterText);

    expect(beforeActual).toEqual(beforeExpectation);
    expect(afterActual).toEqual(afterExpectation);
  }

  // See https://github.com/danvk/github-syntax/issues/17
  test('pure add with assertCharDiff', () => {