JavaScript Fundamentals · Variables & Data Types · Lesson 3 of 48
Declaring Variables with let and const
Concept
Declaring Variables with let and const
Use let for values that change and const for values that should not be reassigned. Avoid var in modern code — it has confusing scoping rules that let and const were introduced to fix.
A good default: reach for const first, and only switch to let when a variable's value genuinely needs to change later.
- Explain the core idea behind Declaring Variables with let and const.
- Predict output before running a change.
- Test one realistic and one unusual input.
- Use the result to make the next decision.
- 1. Read one concept.
- 2. Change the example.
- 3. Run, compare, and explain.
- 4. Complete the challenge below.
let variableName = value;
const constantName = value;let age = 20;
const name = "Alex";
age = 21;
console.log(name, age);const pi = 3.14;
pi = 3.15; // TypeError: Assignment to constant variable.Good to know
const prevents reassignment of the variable itself, but objects and arrays declared with const can still have their contents changed — only the binding is locked.
Self-check before continuing
Without looking at the example, describe what changes when you modify one input in Declaring Variables with let and const. Then reopen the editor and prove your explanation with a small test.
You are ready to continue when you can predict, test, and explain the result.
Your turn
Declare a const for your favorite language and a let for a counter, then log both.
Console