What is the difference between var, let, and const in JavaScript?

Great question! Understanding the differences between `var`, `let`, and `const` is important for writing clean and bug-free JavaScript code. Here’s a breakdown of each and when to use them: ### `var` - **Scope:** Function-scoped. Variables declared with `var` are accessible throughout the entire function they are defined in, or globally if declared outside a function. - **Hoisting:** Variables declared with `var` are hoisted to the top of their scope and initialized with `undefined`. This means you can reference them before the declaration line without a runtime error (though the value will be `undefined`). - **Re-declaration:** You can re-declare and update a `var` variable within the same scope without errors. - **When to use:** `var` is the old way of declaring variables and is generally discouraged in modern JavaScript because of its confusing scoping rules. It is mostly used for legacy code. ### `let` - **Scope:** Block-scoped. Variables declared with `let` are only accessible within the block `{ ... }` they are defined in (such as inside loops, conditionals, or functions). - **Hoisting:** `let` is hoisted but not initialized, so accessing it before declaration causes a `ReferenceError` (this is called the “temporal dead zone”). - **Re-declaration:** You cannot re-declare a `let` variable in the same scope, but you can update its value. - **When to use:** Use `let` when you need a variable whose value will change over time, and you want to limit its scope to a block, avoiding accidental bugs from broader scope leakage. ### `const` - **Scope:** Also block-scoped, just like `let`. - **Hoisting:** Same behavior as `let` regarding hoisting and the temporal dead zone. - **Re-declaration and reassignment:** You cannot re-declare or reassign a `const` variable. The variable must be initialized at the time of declaration. However, if the variable is an object or array, you can still mutate its properties or elements. - **When to use:** Use `const` for variables that should not be reassigned after their initial assignment. This helps make your code safer and more predictable by signaling intent that the binding won’t change. --- ### Summary - Use `const` by default for all variables that don’t need to change. - Use `let` if you expect the variable’s value to change. - Avoid `var` unless working with legacy code or needing function-scoped variables for specific reasons. This approach leads to clearer, more maintainable code and reduces common bugs related to variable scope and accidental reassignment.