Loops in JavaScript

6.1 while Loop

Loops in JavaScript let you run a block of code repeatedly as long as a certain condition holds true. This is a crucial part of programming that helps automate repetitive tasks.

The while loop executes a block of code as long as the condition is true (true).

Syntax:

    
      while (condition) {
        // code that runs while the condition is true
      }
    
  

Example:

    
      let i = 0;
      while (i < 5) {
        console.log(`Iteration number ${i}`);
        i++;
      }

      // Output:
      // Iteration number 0
      // Iteration number 1
      // Iteration number 2
      // Iteration number 3
      // Iteration number 4
    
  

In this example:

• The loop continues as long as the value of i is less than 5
• Inside the loop, the value of i is incremented by 1 after each iteration

6.2 Executing a block of code n times

Quite often you need to execute a block of code a fixed number of times. This can be written in a while loop:

    
      let i = 0;
      while (i < 5) {
        console.log(`Iteration number ${i}`);
        i++;
      }
    
  

In general, it will look like this:

    
      initialization
      while (condition) {
        block of code
        step
      }
    
  

6.3 for Loop

for Loop

The for loop is one of the most commonly used loops in JavaScript. It's specifically designed to repeat a block of code a certain number of times.

Syntax:

    
      for (initialization; condition; step) {
        // code to be executed on each iteration
      }
    
  

Example:

    
      for (let i = 0; i < 5; i++) {
        console.log(`Iteration number ${i}`);
      }
      // Output:
      // Iteration number 0
      // Iteration number 1
      // Iteration number 2
      // Iteration number 3
      // Iteration number 4
    
  

Explanation:

• let i = 0 — initializes the variable i, which will be the iteration counter
• i < 5 — the condition under which the loop continues. The loop stops when i equals 5
• i++ — the step, which is executed after each iteration, incrementing the value of i by 1

6.4 do...while Loop

The do...while loop is similar to the while loop, but with the difference that the block of code is executed at least once before the condition is tested.

Syntax:

    
      do {
        // code to be executed at least once
      } while (condition);
    
  

Example:

    
      let i = 0;
      do {
        console.log(`Iteration number ${i}`);
        i++;
      } while (i < 5);

      // Output:
      // Iteration number 0
      // Iteration number 1
      // Iteration number 2
      // Iteration number 3
      // Iteration number 4
    
  

In this example:

• The block of code inside do will execute at least once, even if the condition is initially false
• After executing the block of code, the condition is tested, and if true, the loop continues

6.5 Comparing Loops

Examples of using loops

Looping through an array with for:

    
      let array = [1, 2, 3, 4, 5];
      for (let i = 0; i < array.length; i++) {
        console.log(array[i]);
      }

      // Output: 1 2 3 4 5
    
  

Using while to wait for a condition:

    
      let n = 0;
      let x = 0;
      while (n < 3) {
        n++;
        x += n;
      }

      console.log(x); // 6 (1 + 2 + 3)
    
  

Using do-while to execute at least one iteration:

    
      let i = 0;

      do {
        console.log(i);
        i++;
      } while (i < 0);

      // Output: 0 (loop executed once despite the false condition)
    
  

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Frontend SELF EN, level 34, lesson 0

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Sum of numbers up to n

Sum of numbers up to n

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Task

Frontend SELF EN, level 34, lesson 0

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Even Numbers Up to 20

Even Numbers Up to 20

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