Type Casting & Coercion

• 22.1 Perform type coercion at the beginning of the statement.

• 22.2 Strings: eslint: no-new-wrappers

  // => this.reviewScore = 9;
  // bad
  const totalScore = new String(this.reviewScore); // typeof totalScore is "object" not "string"
  // bad
  const totalScore = this.reviewScore + ''; // invokes this.reviewScore.valueOf()
  // bad
  const totalScore = this.reviewScore.toString(); // isn’t guaranteed to return a string
  // good
  const totalScore = String(this.reviewScore);

• 22.3 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix no-new-wrappers

  const inputValue = '4';
  // bad
  const val = new Number(inputValue);
  // bad
  const val = +inputValue;
  // bad
  const val = inputValue >> 0;
  // bad
  const val = parseInt(inputValue);
  // good
  const val = Number(inputValue);
  // good
  const val = parseInt(inputValue, 10);

• 22.4 If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you’re doing.

  // good
  /**
   * parseInt was the reason my code was slow.
   * Bitshifting the String to coerce it to a
   * Number made it a lot faster.
   */
  const val = inputValue >> 0;

• 22.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:

  2147483647 >> 0; // => 2147483647
  2147483648 >> 0; // => -2147483648
  2147483649 >> 0; // => -2147483647

• 22.6 Booleans: eslint: no-new-wrappers

  const age = 0;
  // bad
  const hasAge = new Boolean(age);
  // good
  const hasAge = Boolean(age);
  // best
  const hasAge = !!age;