A loop is a programming tool that is used to repeat a set of instructions. Iterate is a generic term that means “to repeat” in the context of loops. A loop will continue to iterate until a specified condition, commonly known as a stopping condition, is met.

For Loop

A for loop declares looping instructions, with three important pieces of information separated by semicolons ;:

• The initialization defines where to begin the loop by declaring (or referencing) the iterator variable.
• The stopping condition determines when to stop looping (when the expression evaluates to false).
• The iteration statement updates the iterator each time the loop is completed.

for (let i = 0; i < 4; i += 1) {
  console.log(i);
}

The output would be:

0
1
2
3

While Loop

The while loop creates a loop that is executed as long as a specified condition evaluates to true. The loop will continue to run until the condition evaluates to false. The condition is specified before the loop, and usually, some variable is incremented or altered in the while loop body to determine when the loop should stop.

while (condition) {
  // Code block to be executed
}

For example:

let i = 0;

while (i < 5) {
  console.log(i);
  i++;
}

The output would be:

0
1
2
3
4

Do…While Loop

A do…while statement creates a loop that executes a block of code once, checks if a condition is true, and then repeats the loop as long as the condition remains true. They are used when the loop body needs to be executed at least once. The loop ends when the condition evaluates to false.

let x = 0;
let i = 0;

do {
  x = x + i;
  console.log(x);
  i++;
} while (i < 5);

The output would be:

0
1
3
6
10

Reverse Loop

A for loop can iterate “in reverse” by initializing the loop variable to the starting value, testing for when the variable hits the ending value, and decrementing (subtracting from) the loop variable at each iteration.

const items = ['apricot', 'banana', 'cherry'];

for (let i = items.length - 1; i >= 0; i -= 1) {
  console.log(`${i}. ${items[i]}`);
}

The output would be:

2. cherry
1. banana
0. apricot