Nested loops in Javascript

Introduction

Nested loops let you repeat actions inside other repeated actions. This helps when you work with things like tables or grids.

When you want to print a grid of stars or numbers.

When you need to check every pair of items in two lists.

When you want to create a multiplication table.

When you process rows and columns in a spreadsheet.

When you want to combine every item from one list with every item from another.

Syntax

Javascript

for (let i = 0; i < outerLimit; i++) {
  for (let j = 0; j < innerLimit; j++) {
    // code to repeat
  }
}

The outer loop runs first, then the inner loop runs completely for each outer loop step.

Use different variable names for each loop to avoid confusion.

Examples

This prints pairs of i and j values for each step of the outer and inner loops.

Javascript

for (let i = 1; i <= 3; i++) {
  for (let j = 1; j <= 2; j++) {
    console.log(`i=${i}, j=${j}`);
  }
}

This example shows how to loop through rows and columns like in a table.

Javascript

for (let row = 1; row <= 2; row++) {
  for (let col = 1; col <= 3; col++) {
    console.log(`Row ${row}, Column ${col}`);
  }
}

Sample Program

This program prints 3 rows of 4 stars each, making a simple star grid.

Javascript

for (let i = 1; i <= 3; i++) {
  let line = '';
  for (let j = 1; j <= 4; j++) {
    line += '* ';
  }
  console.log(line);
}

Important Notes

Nested loops can slow down your program if used with large limits because they repeat many times.

Always check your loop limits to avoid infinite loops.

Summary

Nested loops run one loop inside another to handle repeated tasks in layers.

They are useful for working with grids, tables, or pairs of items.

Remember to use different variables for each loop and watch the limits.