JavaScript Loop For

Loops can execute a block of code a number of times. They are perfect for situations where you need to run the same code over and over again, each time with a different value.

The For Loop Syntax

The for loop has three optional expressions, separated by semicolons:

for (expression 1; expression 2; expression 3) {
  // code block to be executed
}

The Three Expressions

• Expression 1 is executed (one time) before the starting of the code block. Generally used to initialize the counter variable.
• Expression 2 defines the condition for executing the code block. If it returns true, the loop starts over; if it returns false, the loop ends.
• Expression 3 is executed (every time) after the code block has been executed. Generally used to increment the counter.

Basic Example

A simple loop that counts from 0 to 4:

for (let i = 0; i < 5; i++) {
  console.log("The number is " + i);
}

Looping Through an Array

For loops are most commonly used to iterate over arrays.

const cars = ["BMW", "Volvo", "Saab", "Ford"];

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

Skipping Expressions

Expressions in a for loop are optional. For example, you can omit the first expression if your values are already set:

let i = 2;
let len = cars.length;
for (; i < len; i++) {
  text += cars[i];
}

Key Points to Remember

• Loops save time and reduce code redundancy
• The for loop is the most versatile looping structure
• All three expressions are optional, but semicolons are required
• Block scope (using let) is the best practice for loop counters
• The condition is checked before every iteration
• The increment/decrement happens after the code block runs

Common Mistakes to Avoid

Try It: Sum and Average of a Price List

A second example that does real work with the loop instead of just printing values:

const prices = [12.50, 8.75, 20.00, 15.25];
let total = 0;

for (let i = 0; i < prices.length; i++) {
    total += prices[i];
}

let average = total / prices.length;
console.log("Total: " + total + ", Average: " + average.toFixed(2));

Frequently Asked Questions