[JavaScript] Iterate

While Loops

var myArr = [];
var i = 0;
 
while(i < 5) {
  myArr.push(i);
  i++;
}

For Loops

var myArr = [];
 
// for ([initialization]; [condition]; [final-expression])
for (var i = 0; i < 5; i++) {
  myArr.push(i);
}

Iterate through an Array with a For Loop

var myArr = [ 2, 3, 4, 5, 6];
var total = 0;
 
for (var i = 0; i < myArr.length; i++) {
    total += myArr[i];
}

Nesting For Loops

var myArr = [
  [1,2], [3,4], [5,6]
];
 
for (var i=0; i < myArr.length; i++) {
  for (var j=0; j < myArr[i].length; j++) {
    console.log(myArr[i][j]);
  }
}

For…In Loops

Iterate through all the keys within an object.

let users = {
  Max: {
    age: 27
  },
  Mira: {
    age: 32
  },
  Rich: {
    age: 48
  }
};
 
for (let user in users) {
  console.log(user); // logs: Max, Mira, Rich
  if (users[user].age > 40) {
      console.log(`${user} is old.`);
  }
} 

Do…While Loops

A do...while loop ensures that the code inside the loop will run at least once.

var myArr = [];
var i = 0;
do {
  myArr.push(i);
  i++;
} while (i < 5);

Replace Loops using Recursion

/* For Loop */
function multiply(arr, n) {
   var product = 1;
   for (var i = 0; i < n; i++) {
       product *= arr[i];
   }
   return product;
}
 
/* Replace For Loop with Recursion */
function multiply(arr, n) {
   if (n <= 0) {
     return 1;
   } else {
     return multiply(arr, n - 1) * arr[n - 1];
   }
}

Note: Recursive functions must have a base case when they return without calling the function again (in this example, when n <= 0), otherwise they can never finish executing.

/* Count to n */
function countup(n) {
  if (n < 1) {
    return [];
  } else {
    const countArray = countup(n - 1);
    countArray.push(n);
    return countArray;
  }
}
 
console.log(countup(5)); // [ 1, 2, 3, 4, 5 ]

Note: The push happens last, after the recursive call has returned. Thats why the value of n decreases, but the values in the final array are increasing.

/* Create a Range of Numbers */
function rangeOfNumbers(startNum, endNum) {
  if (startNum == endNum) {
    return [startNum];
  } else if (startNum < endNum) {
    const rangeArray = rangeOfNumbers(startNum + 1, endNum);
    rangeArray.unshift(startNum);
    return rangeArray;
  } 
};
 
console.log(rangeOfNumbers(5, 10)); // [ 5, 6, 7, 8, 9, 10 ]

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