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

Great question! Understanding the differences between `let`, `const`, and `var` is essential for writing clear and bug-free JavaScript code. ### `var` `var` is the older way to declare variables in JavaScript (introduced in ES5 and before). It has **function scope**, meaning a variable declared with `var` is scoped to the entire function it’s declared in, or global if declared outside any function. Additionally, `var` declarations are **hoisted**, which means the declaration is moved to the top of its scope during compilation, but not the assignment. This can sometimes lead to confusing bugs. Also, `var` does **not** have block scope, so if you declare a `var` variable inside a block (like inside an `if` or `for`), it's actually accessible outside that block, which can cause unintended side effects. ### `let` Introduced in ES6 (ES2015), `let` provides **block scope**, meaning the variable is limited to the block, statement, or expression where it is declared (e.g., inside `{}` curly braces). Unlike `var`, `let` is not hoisted in a way that allows access before declaration (it’s in a "temporal dead zone"), which helps avoid bugs caused by premature use. Use `let` when you expect the variable value to change over time, such as counters in loops or values that need reassignment. ### `const` Also introduced in ES6, `const` is similar to `let` in terms of block scope and temporal dead zone. The key difference is that a variable declared with `const` **cannot be reassigned** after its initial assignment. This makes `const` ideal for declaring constants or values you know will not change, improving code safety and clarity. Note that `const` does **not** make objects or arrays immutable—it only prevents reassigning the variable itself. You can still modify properties of a `const` object or elements of a `const` array. ### When to use each: - Use `const` by default for all variables, as it signals that the variable should not be reassigned. - Use `let` if you know the variable’s value needs to change (e.g., loop counters, temporary values). - Avoid using `var` in modern JavaScript because its function scope and hoisting behavior are often sources of bugs and confusion. It’s mainly kept for legacy code support. ### Summary: | Keyword | Scope | Reassignable? | Hoisting Behavior | Typical Use Case | |---------|--------------|---------------|---------------------------------|---------------------------| | var | Function | Yes | Hoisted, initialized as undefined | Legacy code, rarely used | | let | Block | Yes | Hoisted, but in temporal dead zone | Variables that change | | const | Block | No (reassignment) | Hoisted, but in temporal dead zone | Constants, fixed references | By following this guidance, your code will be easier to read, maintain, and less prone to subtle bugs.