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[Hard] πŸ“„ IIFE

1. What's the IIFE ?​

IIFE stands for Immediately Invoked Function Expression.
Compared with a normal function declaration, it wraps the function with an extra () and executes immediately:

(() => {
  console.log(1);
})();

# or

(function () {
  console.log(2);
})();

It can also run repeatedly through recursion until a stop condition is reached, and the trailing () can pass in parameters.

(function myIIFE() {
  num += 1;
  console.log(num);
  return num < 5 ? myIIFE(num) : console.log('finished!!');
})((num = 0));

Note that an IIFE runs at initialization time (or via internal self-calls), but cannot be called again directly from outside.

2. Why use IIFE ?​

scope​

Because variables declared inside a function are scoped to that function, IIFE can isolate state and avoid polluting globals:

// global
const name = 'Yumi';
const Hello = () => {
  return `Hello ${name}!`;
};

(() => {
  const name = 'Pitt';
  const Hello = () => {
    return `Hello ${name}!`;
  };
  console.log(name); // result Pitt
  console.log(Hello()); // result Hello Pitt!
})();

console.log(name); // result Yumi
console.log(Hello()); // result Hello Yumi!

private variable and methods​

Using IIFE with closure can create private variables and methods.
That means state can be preserved inside the function and updated on each call (for example, increment/decrement).

const increment = (() => {
  let result = 0;
  console.log(result);
  const credits = (num) => {
    console.log(`I have ${num} credits.`);
  };
  return () => {
    result += 1;
    credits(result);
  };
})();

increment(); // I have 1 credits.
increment(); // I have 2 credits.

Be careful: because those variables persist, overuse can consume memory and hurt performance.

module​

You can expose functionality in object form as a module pattern.
In the example below, you can increment and also reset state:

const Score = (() => {
  let result = 0;

  return {
    current: () => {
      return result;
    },

    increment: () => {
      result += 1;
    },

    reset: () => {
      result = 0;
    },
  };
})();

Score.increment();
console.log(Score.current()); // result 1 => 0 + 1 = 1
Score.increment();
console.log(Score.current()); // result 2 => 1 + 1 = 2
Score.reset();
console.log(Score.current()); // result 0 => reset = 0

Another style:

const Score = (() => {
  let result = 0;
  const current = () => {
    return result;
  };

  const increment = () => {
    result += 1;
  };

  const reset = () => {
    result = 0;
  };

  return {
    current: current,
    increment: increment,
    reset: reset,
  };
})();

Score.increment();
console.log(Score.current());
Score.increment();
console.log(Score.current());
Score.reset();
console.log(Score.current());

One more note: because IIFEs execute immediately, placing two IIFEs back-to-back can break ASI (Automatic Semicolon Insertion).
When chaining IIFEs, add semicolons explicitly.