Control Flow: Repeating with Loops

Learn how to repeat actions using for, while, and do...while loops

Loops let you execute a block of code multiple times without writing it over and over.

The for Loop

Use a for loop when you know how many times to repeat:

javascript

for (let i = 0; i < 5; i++) {
  console.log("Iteration:", i);
}

A for loop has three parts:

Initialization: let i = 0: runs once before the loop
Condition: i < 5: checked before each iteration
Update: i++: runs after each iteration

The while Loop

Use while when you want to repeat until a condition changes:

javascript

let count = 0;

while (count < 5) {
  console.log("Count:", count);
  count++;
}

⚠️Warning

If the condition never becomes false, the loop runs forever and crashes your program!

The do...while Loop

Similar to while, but the body always runs at least once:

javascript

let i = 0;

do {
  console.log("This runs at least once, i =", i);
  i++;
} while (i < 3);

break and continue

break exits the loop immediately
continue skips to the next iteration

javascript

for (let i = 0; i < 10; i++) {
  if (i === 3) {
    continue;  // Skip 3
  }
  if (i === 7) {
    break;     // Stop at 7
  }
  console.log(i);  // Output: 0, 1, 2, 4, 5, 6
}

Looping Through Arrays

A common pattern is iterating over array elements:

javascript

let fruits = ["Apple", "Banana", "Cherry"];

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

JavaScript also offers a cleaner syntax:

javascript

for (let fruit of fruits) {
  console.log(fruit);
}

Nested Loops

Loops can be placed inside other loops:

javascript

for (let row = 0; row < 3; row++) {
  let line = "";
  for (let col = 0; col < 3; col++) {
    line += "* ";
  }
  console.log(line);
}
// Output:
// * * *
// * * *
// * * *

Practice Exercise

Write a function that calculates the sum of numbers from 1 to n:

javascript

function sumTo(n) {
  let total = 0;
  for (let i = 1; i <= n; i++) {
    total += i;
  }
  return total;
}

console.log(sumTo(5));   // Output: 15  (1+2+3+4+5)
console.log(sumTo(100)); // Output: 5050

Summary

Loop	When to use
`for (init; cond; update)`	Known number of iterations
`while (cond)`	Run while condition is true
`do { ... } while (cond)`	Run at least once
`for...of`	Iterate over array elements
`break`	Exit loop early
`continue`	Skip current iteration

Next Steps

Now that you can repeat actions, the next lesson covers data structures: arrays and objects for organizing data.

Progress56%

Control Flow: Making Decisions with Conditionals

Data Structures: Arrays and Objects

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Course Content

1. Getting Started with Programming 2. Understanding Variables 3. Functions: Reusable Code Blocks 4. Control Flow: Making Decisions with Conditionals 5. Control Flow: Repeating with Loops 6. Data Structures: Arrays and Objects 7. Introduction to Functional Programming with Python 8. Compiled Programming Languages with Rust 9. Declarative Programming: SQL and HCL