Object.entries()

The Object.entries() method returns an array of a given object's own enumerable string-keyed property [key, value] pairs, in the same order as that provided by a for...in loop. (The only important difference is that a for...in loop enumerates properties in the prototype chain as well).

The order of the array returned by Object.entries() does not depend on how an object is defined. If there is a need for certain ordering, then the array should be sorted first, like Object.entries(obj).sort((a, b) => b[0].localeCompare(a[0]));.

for-ofloop

const object1 = {
  a: 'somestring',
  b: 42
};

for (const [key, value] of Object.entries(object1)) {
  console.log(`${key}: ${value}`);
}

// expected output:
// "a: somestring"
// "b: 42"
// order is not guaranteed

for-in loop

const myObject = {
    a: 'somestring',
    b: 42
};

for (const [key, value] of Object.entries(myObject)) {
    for (const keyItem in key) {
        console.log(key[keyItem]); // Expected: a & b
    }
}

Practical example with for, for-in and for-of

function printLogHandler() {
  for (let i = 0; i < 3; i++) {
    console.log('------------');
  }
  let i = 0;
  for (const logEntry of battleLog) {
    console.log(`#${i}`);
    for (const key in logEntry) {
      console.log(`${key} => ${logEntry[key]}`);
    }
    i++;
  }
}

// Expected output
// #0
// event => PLAYER_ATTACK
// value => 0.33673722139444084
// finalMonsterHealth => 99.66326277860556
// finalPlayerHealth => 100
// target => MONSTER
// #1
// event => MONSTER_ATTACK
// value => 11.435543505902086
// target => PLAYER
// finalMonsterHealth => 99.66326277860556
// finalPlayerHealth => 88.56445649409791
// #2
// event => PLAYER_ATTACK
// value => 2.052553803920303
// finalMonsterHealth => 97.61070897468525
// finalPlayerHealth => 88.56445649409791
// target => MONSTER

while loop with an example

(function () {
    let randomNumbers = [];
    let finished;
    while (!finished) {
        const rndNumber = Math.random();
        randomNumbers.push(rndNumber);
        if (rndNumber > 0.5) {
            finished = true;
            console.log(randomNumbers);
        }
    }
})();
// This example uses a IIFE

Question

What's the main difference between for-of and for-in loops?

Answer: for-of are used for arrays and for-in for objects

break vs return

break is used when you want to exit from the loop, while return is used to go back to the step where it was called or to stop further execution.

(function () {
    const myArray = [1, 2, 3, 4, 5, 6];
    for (let i = 0; i < myArray.length; i++) {
        if (myArray[i] === 3) {
            console.log('This uses return');
            return;
        }
    }

    for (let i = 0; i < myArray.length; i++) {
        if (myArray[i] === 3) {
            console.log('This uses break');
            break;
        }
    }
})();

// Expected:
// This uses return
// This will never get to the break statement

(function () {
    const myArray = [1, 2, 3, 4, 5, 6];
    // for (let i = 0; i < myArray.length; i++) {
    //     if (myArray[i] === 3) {
    //         console.log('This uses return');
    //         return;
    //     }
    // }

    for (let i = 0; i < myArray.length; i++) {
        if (myArray[i] === 3) {
            console.log('This uses break');
            break;
        }
    }
})();
// Expected result: 
// This uses break