JS Loop For

JavaScript for Loop: Iteration, Counters, & Nesting

The for loop executes a block of code repeatedly until a specified condition evaluates to false. It consists of three optional expressions: initialization, condition check, and final increment/decrement expression.

for Loop Execution Lifecycle

flowchart TD
    Init["1. Initialization (let i = 0)"] --> Condition{"2. Condition Check (i < max)?"}
    Condition -- "True" --> Body["3. Execute Loop Body Block"]
    Body --> Inc["4. Increment / Update (i++)"] --> Condition
    Condition -- "False" --> Exit["5. Exit Loop"]

for Loop Structure Anatomy

// for (initialization; condition; increment)
for (let i = 0; i < 5; i++) {
  // Loop body statement
}

Practical Code Example

// Demonstrating standard for loops, reverse iteration, and matrix processing

// 1. Standard Forward Loop
console.log("--- Forward Loop ---");
for (let i = 1; i <= 3; i++) {
  console.log(`Count: ${i}`);
}

// 2. Reverse Loop (Counting down)
console.log("
--- Countdown Loop ---");
for (let i = 3; i > 0; i--) {
  console.log(`Countdown: ${i}`);
}

// 3. Nested Loop (Matrix / Grid Processing)
console.log("
--- 2D Matrix Iteration ---");
const grid = [
  [1, 2],
  [3, 4]
];

for (let row = 0; row < grid.length; row++) {
  for (let col = 0; col < grid[row].length; col++) {
    console.log(`Grid[${row}][${col}] = ${grid[row][col]}`);
  }
}

// 4. Multiple Counters in Single Loop
for (let i = 0, j = 10; i < 3; i++, j -= 2) {
  console.log(`i=${i}, j=${j}`);
}

Best Practices & Gotchas

• Declare Counter with let: Always declare loop counters with let (for (let i = 0; ...)) so each iteration receives a block-scoped binding.
• Cache Array Length for High Performance: In performance-critical loops processing massive arrays, cache length: for (let i = 0, len = arr.length; i < len; i++).
• Avoid Infinite Loops: Ensure the loop condition eventually evaluates to false and the increment updates the counter correctly.

Self-Check Challenge

Write a for loop that prints all even numbers from 0 to 20 in steps of 2 (i += 2).