Understanding Loops in Javascript

A loop is to iterate(repeat) over a set of code until a specified condition is reached.

The article does the following:

Explain How tasks can be manually repeated
Explain For Loop
Explain Looping in reverse
Explain nested loops.
explain while loops.
Explain Do...While statements.
Explain the break keyword.

Before we start to work with loops, we will see how cumbersome having to write a repeated task can be:



const myNames = ['Adeleye', 'Oluwaloseyi', 'Jamiu'];

console.log(myNames[0]);
console.log(myNames[1]);
console.log(myNames[2]);

/*Outputs:
Adeleye
Oluwaloseyi
Jamiu
*/

The example above might not look so cumbersome as it only involves three values. Imagine we had over one hundred values. It would be easier to use loops than have to console.log each value.

A loop is best used when a process has to be repeated multiple times in a row. Loops allow the creation of efficient code that automates processes to make scalable, manageable programs.

The `For` Loop

Instead of writing out the same code over and over, loops allow computers to repeat a given block of code on its own. One way to give computers these instructions is with a for loop.

A for loop contains three expressions separated by ; inside the parentheses:

An iterator variable is initialized which starts the loop.
The iterator variable is checked against a stopping condition. If the condition evaluates to true the code block will run, and if it evaluates to false the code will stop.
An iteration statement is used to update the iterator variable on each loop.

Iterator variable can have any name, but it’s best practice to use a descriptive variable name.

from the example above where we manually logged our names we can instead use a for loop like below:



const myNames = ['Adeleye', 'Oluwaloseyi', 'Jamiu'];

for (let i = 0; i < myNames.length; i++) {
  console.log(myNames[i]);
}

/*Outputs:
Adeleye
Oluwaloseyi
Jamiu
*/

We notice that we get the same output as when we manually repeat the codes. Even if we have over a hundred values for myNames, we would only need to write the same line of code.

From the code example above

The initialization is let i = 0 and the iterator variable is i, so the loop will start counting at 0.
The stopping condition is i < myNames.length, meaning the loop will run as long as the iterator variable, i, is less than the length of myNames variable.
The iteration statement is i++. This means that after each loop, the value of counter will increase by 1. For the first iteration i will equal 0, for the second iteration i will equal 1, and so on.
The code block is inside of the curly braces, console.log(myNames[i]), will execute until the condition evaluates to false. The condition will be false when i is greater than or equal to myNames.length.

Looping in Reverse

With simple modifications to the expressions, the for loop can be made to run backward and display results in reverse.

To run a backward for loop:

Set the iterator variable to the highest desired value in the initialization expression.
Set the stopping condition for when the iterator variable is less than the desired amount.
The iterator should decrease in intervals after each iteration.



const myNames = ['Adeleye', 'Oluwaloseyi', 'Jamiu'];

for (let i = myNames.length; i >= 0; i--) {
  console.log(myNames[i]);
}

/*Outputs:
Jamiu
Oluwaloseyi
Adeleye
*/

Nested Loops

Nested is when there exists a loop running inside another loop. This is useful when comparing the elements of two arrays. For each round of the outer for loop, the inner for loop will run completely.



const firstNumbers = [6, 19, 20];
const secondNumbers = [19, 81, 2];
for (let i = 0; i < firstNumbers.length; i++) {
  for (let j = 0; j < secondNumbers.length; j++) {
    if (firstNumbers[i] === secondNumbers[j]) {
      console.log('Both loops have the number: ' + secondNumbers[j]);
    }
  }
} //OutPuts: Both loops have the number: 19

For each element in the outer loop array, firstNumbers, the inner loop will run in its entirety comparing the current element from the outer array, firstNumbers[i], to each element in the inner array, secondNumbers[j]. When it finds a match, it prints a string to the console.

The While Loop

A for loop can be converted into a while loop:



// A for loop that prints 1, 2, and 3
for (let counterOne = 1; counterOne < 4; counterOne++) {
  console.log(counterOne);
}
// A while loop that prints 1, 2, and 3
let counterTwo = 1;
while (counterTwo < 4) {
  console.log(counterTwo);
  counterTwo++;
}

Breaking down what’s happening with the while loop syntax:

The counterTwo variable is declared before the loop which can be accessed inside the while loop since it’s in the global scope.
We start the loop with the keyword while followed by the stopping condition, or test condition. This will be evaluated before each round of the loop. While the condition evaluates to true, the block will continue to run. Once it evaluates to false the loop will stop.
Next, we have the loop’s code block which prints counterTwo to the console and increments counterTwo.

If we didn’t increment counterTwo, it would always have its initial value, 1. That would mean the testing condition counterTwo < 4 would always evaluate to true and the loop would never stop running(infinite loop). Infinite loops can take up all of our computer’s processing power potentially freezing our computer and it’s something we always want to avoid.

The syntax of a for loop is ideal when the number of times the loop should run is known, but this is not always known in advance. While In situations when there is a need for a loop to execute an undetermined number of times, while loops are the best choice.

`Do...While` Statements

do...while statement comes in is a case where there is a need to run a piece of code at least once and then loop based on a specific condition after its initial run.

A do...while statement says to do a task once and then keep doing it until a specified condition is no longer met.



let countInt = 0;
let i = 0;
do {
  countInt = countInt + i;
  i++;
} while (i < 5);
console.log(countInt);

// Outputs: 10

In this example, the code block makes changes to the countInt variable by adding i variable to it. First, the code block after the do keyword is executed once. Then the condition is evaluated. If the condition evaluates to true, the block will execute again. The looping stops when the condition evaluates to false.

Note that the while and do...while loop is different. Unlike the while loop, do...while will run at least once whether or not the condition evaluates to true.



const firstChoice = 'Soccer!';
const secondChoice = 'Rugby!';
// A do while with a stopping condition that evaluates to false
do {
  console.log(firstChoice);
} while (true === false);
// A while loop with a stopping condition that evaluates to false
while (true === false) {
  console.log(secondChoice);
}

//Outputs: Soccer!

The results only output Soccer!.

The break Keyword

The break keyword is used to break away from a loop after a certain condition has been met even if the stopping condition has not been met.

The break keyword allows programs to “break” out of the loop from within the loop’s block.



for (let i = 0; i < 99; i++) {
  if (i > 2) {
    break;
  }
  console.log('Am still in the loop.');
}
console.log('Glad I broke out of the loop!');
// This is the output for the above code:
/*
Am still in the loop.
Am still in the loop.
Am still in the loop.
Glad I broke out of the loop!
*/

We notice that we were able to break out of the loop even as the stopping condition i < 99 is still true. break statements can be especially helpful when looping through large data structures. With breaks, we can add test conditions besides the stopping condition, and exit the loop when they’re met.