Start with the question: will this variable be reassigned?
In JavaScript, a variable gives a name to a value so you can use it later. The two keywords you will usually choose between are const and let. Their most useful difference is whether you can assign a new value to that name: const prevents reassignment, while let allows it. A practical default is to use const, then switch to let when the code needs to reassign the variable.
You can try the examples in a browser developer console or a Node.js environment that supports ECMAScript 2015 (ES6) or newer. No library or project setup is required. Run each example separately so an intentional error in one example does not interrupt the others.
For example, const greeting = "Hello"; creates a variable whose value cannot be reassigned. The statement greeting = "Hi"; then produces an error. By contrast, let greeting = "Hello"; followed by greeting = "Hi"; is allowed. In both cases, JavaScript uses the name greeting to access the value currently assigned to it.
Use const when the name should keep its value
Choose const for values that a particular variable name will not need to replace. For example, const pageTitle = "My First Page"; and const maxAttempts = 3; make that intention clear. The value does not have to be a universal constant: it can be calculated when the code runs. In const total = price + tax;, total is assigned once, even if price and tax came from earlier calculations.
A const declaration needs an initial value. const score; is an error because there is nothing to assign to score. If you do not have a value yet, consider whether you can move the declaration to the point where the value is available. If you genuinely need to declare the name first and assign it later, use let.
Importantly, const protects the assignment to the variable, not every part of the value. After const settings = { theme: "light" };, the statement settings.theme = "dark"; can change a property of the object. But settings = { theme: "dark" }; tries to assign a different object to settings and produces an error. Arrays behave similarly: you can add an item to an array declared with const, but you cannot reassign that variable to another array. If you need an object whose properties cannot be changed, const alone does not provide that guarantee.
Use let for a value that must change
Choose let when reassignment is part of the job. A counter is a straightforward example: let attempts = 0; followed later by attempts = attempts + 1;. The name attempts represents the current count, so replacing its value is expected. A variable holding the current step of a process or a result that is updated as conditions are checked may also need let.
Let can be declared without an initial value: let message;. Until it receives an assignment, its value is undefined. That is useful occasionally, but leaving a variable unset can make code harder to follow. When possible, declare it near its first assignment and give it a meaningful starting value.
Do not choose let merely because the value is an object that you plan to edit. If the variable will keep referring to the same object, const still works. The decision is about reassigning the variable name, not about whether the underlying data might change.
Remember that both keywords have block scope
A block is code enclosed in braces, such as the body of an if statement or a loop. Variables declared with let or const inside a block are available within that block, not outside it. For example, in if (true) { const note = "Ready"; }, using note after the closing brace produces an error because note was declared inside the block.
Block scope helps keep temporary variables close to the code that needs them. It also means separate blocks can use the same variable name without referring to the same variable. If you need a result after a block finishes, declare a variable in the surrounding scope first, or structure the code so the value can be returned from a function.
Both let and const also have a period before their declaration during which accessing them produces an error. As a beginner-friendly rule, declare each variable before you use it. You may encounter the older keyword var in existing code, but its scoping behavior differs; for new examples, let and const usually make your intent easier to see.
Apply a simple decision rule
When writing a new variable declaration, ask: “Will this name need a new assigned value later?” If the answer is no, use const. If the answer is yes, use let. If you are unsure, start with const; a reassignment error will show you where let may be appropriate. Then check whether the reassignment is actually needed or whether a new, more descriptive variable would make the code clearer.
For instance, const input = " hello "; and const cleanedInput = input.trim(); give distinct names to the original and cleaned strings. You could instead write let input = " hello "; and then input = input.trim();. Both approaches work, but separate const variables can be easier to follow when you need both values. The goal is not to avoid let at all costs. It is to make changes to a variable deliberate and understandable.
