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In TypeScript, let and const are both block-scoped declarations. The key difference is whether you can assign a new value to the variable: use let when reassignment is part of the logic, and const when it is not. A const binding does not make an object or array immutable.
How are let and const different?
| Declaration | Can the binding be reassigned? | Scope | What happens to object contents? |
|---|---|---|---|
let |
Yes | Block scope | Contents can be changed if the value is mutable. |
const |
No, after initialization | Block scope | Contents can still be changed if the referenced value is mutable. |
The TypeScript Handbook describes let and const as block-scoped, so neither declaration is visible outside the block in which it appears. This differs from var, which is function-scoped. See the Handbook’s variable declaration guide.
When should you use each one?
Use const when you do not need to reassign the variable
The TypeScript Handbook recommends that declarations you do not plan to modify use const. This is guidance, not a syntax requirement; it makes the intended data flow clearer by preventing accidental reassignment.
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const taxRate = 0.08;
// taxRate = 0.1; // Error: cannot reassign a const binding
Use let when reassignment is intentional
If your logic updates a variable with a different value, declare it with let.
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let total = 0;
total += 12; // Reassignment is allowed with let
Does const make an object or array immutable?
No. const protects the binding—the name cannot be made to refer to a different value. It does not freeze the object or array the binding refers to. Mutable properties can still be changed:
const settings = { theme: "light" };
settings.theme = "dark"; // Allowed: the binding has not changed
// settings = { theme: "dark" }; // Error: cannot reassign the binding
This distinction is useful when deciding what protection your code needs: const prevents rebinding, while property or collection restrictions require type-level constructs such as readonly or as const.
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How do readonly and as const differ from const?
readonly restricts properties or collections in the type
Use a readonly property when code should not assign a new value to that property through the declared type. For example:
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readonly y: number;
}
const point: Point = { x: 1, y: 2 };
// point.x = 3; // TypeScript error: x is readonly
The variable declaration uses const; the interface’s readonly modifiers restrict assignments to the properties at the TypeScript type level. The Handbook sums up the distinction: “Variables use const whereas properties use readonly.” See TypeScript’s Interfaces Handbook.
as const preserves literal types and readonly literal structure
A const assertion is a type assertion on a literal expression, not a variable declaration. TypeScript 3.4 introduced as const to prevent literal widening; object-literal properties become readonly and array literals become readonly tuples.
const palette = { primary: "blue" } as const;
// primary is readonly and has the literal type "blue"
Neither readonly nor as const should be understood as runtime freezing or a guarantee of deep immutability. The TypeScript 3.4 release notes show that referenced mutable values can still be changed.
What does this mean at runtime?
TypeScript is a static typechecker for JavaScript that checks code before it runs, as explained in the TypeScript Handbook introduction. The practical choice between let and const controls whether the binding can be reassigned; it does not add a TypeScript-specific runtime object-freezing guarantee.
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