Variables

JavaScript has three variable declaration keywords: var (original), let and const (added in ES6/ES2015). Understanding their differences in scope, hoisting, and mutability is fundamental and one of the most common interview questions.

var

• Function-scoped: a var declared inside a function is accessible anywhere within that function, regardless of block boundaries (if, for, etc.).
• If declared outside any function, var creates a global variable and adds a property to the global object (window in browsers).
• Can be re-declared in the same scope without error: var x = 1; var x = 2; is valid.
• Hoisted and initialized to undefined — accessing before declaration returns undefined instead of throwing an error.
• This behavior causes subtle bugs, especially in loops: a var in a for loop is shared across all iterations, which breaks closures inside the loop.

let

• Block-scoped: limited to the nearest enclosing block ({} — if, for, while, or any curly-brace block).
• Cannot be re-declared in the same scope — let x = 1; let x = 2; throws SyntaxError.
• Can be reassigned: let x = 1; x = 2; is valid.
• Hoisted but not initialized — accessing before declaration throws ReferenceError (Temporal Dead Zone).
• Can be declared without initialization: let x; is valid (value is undefined until assigned).

const

• Block-scoped, same as let.
• Must be initialized at declaration: const x; throws SyntaxError.
• Cannot be reassigned: const x = 1; x = 2; throws TypeError.
• Important nuance: const prevents reassignment of the binding, not mutation of the value. A const object or array can still have its properties/elements modified: const arr = [1]; arr.push(2); is valid.
• To make an object truly immutable, use Object.freeze() — but this is shallow (nested objects are still mutable).

Best Practices

• Use const by default — it signals that the binding won't change, making code easier to reason about.
• Use let only when you need to reassign (loop counters, accumulators, conditional assignments).
• Avoid var entirely in modern code — it causes scoping bugs and is hoisted in confusing ways.
• Declare variables as close to their usage as possible.

Key Interview Distinction: const Does Not Mean Immutable const prevents reassignment of the variable binding, not mutation of the value. const obj = {}; obj.key = 'value'; works fine — you're mutating the object, not reassigning obj. This is the most commonly misunderstood aspect and a frequent interview trick question.