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In JavaScript, a variable declaration creates a named binding; the data type belongs to the value currently held by that binding. Use const when you do not need to assign the binding again, let when you do, and learn var mainly to understand older code. JavaScript is dynamically typed, so a binding can hold values of different types over time.
What is a variable in JavaScript?
A variable is a binding between a name and a value. For example, let score = 10; declares the name score and initializes it with the number 10. The declaration is about how the name can be used; the type describes the value it currently refers to.
A declaration without an initializer, such as let count;, leaves the binding with the value undefined until a value is assigned. A const declaration must include an initializer.
How do let, const, and var differ?
The main practical differences are whether a binding can be reassigned, where it is visible, and whether it must be initialized immediately.
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| Declaration | Scope | Can the binding be reassigned? | Initializer required? |
|---|---|---|---|
let |
Block scope | Yes | No |
const |
Block scope | No | Yes |
var |
Enclosing variable environment; a block alone does not contain it | Yes | No |
A block is code enclosed in braces, such as an if body or a loop body. let and const are visible only within the block where they are declared. The ECMAScript 2026 specification states that these declarations are scoped to the execution context’s lexical environment (ECMAScript declarations).
let score = 10;
score = 11; // reassignment is allowed
if (true) {
let blockValue = "inside";
var functionValue = "visible outside this block";
}
// blockValue is not available here
// functionValue is available hereA let or const binding also cannot be accessed before its initialization point; the period before initialization is called the temporal dead zone. var behaves differently because its scope is tied to the surrounding variable environment rather than limited to the nearest block. MDN notes that this behavior can be surprising and discourages using var in modern code (MDN: JavaScript language overview).
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When should you choose each declaration?
- Choose
constwhen the name should keep referring to the same value. It is a good default when reassignment is unnecessary. - Choose
letwhen your code needs to assign a new value to the binding. - Recognize
varwhen reading existing code, especially when its visibility across a block matters.
Does const make an object immutable?
No. const prevents reassignment of the binding, but it does not freeze the object or array that the binding refers to. You can change an object’s properties while keeping the same binding:
const settings = { theme: "light" };
settings.theme = "dark"; // allowed
// settings = {}; // TypeError: reassignment is not allowedThis distinction is covered in MDN’s const reference.
What are JavaScript data types?
A data type is a category of value. ECMAScript defines seven primitive types and the Object type (ECMAScript 2024 language types).
undefined: commonly the value of a declared binding that has not been assigned another value.null: an intentional absence of an object value.boolean: eithertrueorfalse.number: ordinary numeric values.bigint: integer values for use cases beyond ordinaryNumbervalues.string: text.symbol: a unique symbol value.object: structured values. Objects and arrays are objects; functions are also objects in the language model.
The primitive-versus-object distinction is part of JavaScript’s language model. Arrays and functions may be treated as separate practical categories in everyday discussion, but they are objects in that classification.
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Can a variable change its data type?
Yes. JavaScript is dynamically typed: a binding is not permanently restricted to the type of its first value. The type describes the value currently assigned to it.
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let value = 42;
console.log(typeof value); // "number"
value = "hello";
console.log(typeof value); // "string"Here, the name value remains the same, but its current value—and therefore its type—changes. MDN explains this distinction in its JavaScript language overview.
How do you use typeof?
The typeof operator returns a string indicating the type category of a value. It is useful for inspecting simple values while learning:
typeof 500; // "number"
typeof "500"; // "string"
typeof true; // "boolean"
typeof undefined; // "undefined"Quotation marks matter: "500" is text, not a number. typeof inspects the value it receives; it does not reveal a permanent type attached to the variable name. MDN’s beginner guide demonstrates this use of typeof (MDN: Variables).
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