[TS/JS] Logical Assignment Operatorの挙動

挙動のメモ書き

• TS Playground

/**
 * Logical Assignment Operators
 */

{
  /**
   * Logical AND assignemnt
   * 
   * @link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_AND_assignment
   */
  const log = (...args: any[]) => console.log("#AND", ...args);

  let a1 = undefined;
  a1 &&= "this is NOT new value"; // 🙅‍♀️
  log({ a1 });

  let b1 = "this is prev value";
  b1 &&= "this is new value"; // 🙆‍♂️
  log({ b1 });

  let c1 = "";
  c1 &&= "this is NOT new value"; // 🙅‍♀️
  log({ c1 });
}

{
  /**
   * Logical OR assignemnt
   * 
   * @link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR_assignment
   */
  const log = (...args: any[]) => console.log("#OR", ...args);

  let a1 = undefined;
  a1 ||= "this is new value" // 🙆‍♂️
  log({ a1 });

  let b1 = "this is prev value";
  b1 ||= "this is NOT new value"; // 🙅‍♀️
  log({ b1 });

  let c1 = "";
  c1 ||= "this is new value"; // 🙆‍♂️
  log({ c1 });
}

{
  /**
   * Nullish coalescing assignment
   * 
   * @link 
   */
  const log = (...args: any[]) => console.log("#NULLISH", ...args);

  let a1 = undefined;
  a1 ??= "this is new value" // 🙆‍♂️
  log({ a1 });

  let b1 = "this is prev value";
  b1 ??= "this is NOT new value"; // 🙅‍♀️
  log({ b1 });

  let c1 = "";
  c1 ??= "this is NOT new value"; // 🙅‍♀️
  log({ c1 });
}