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In TypeScript, both let and const declare block-scoped variables. The key difference is reassignment: use let when the variable must later hold a different value; use const when its binding should stay fixed. A const binding does not, by itself, make an object or array immutable.
How do let and const differ?
let allows reassignment after initialization. const requires an initial value and prevents the declared name from being assigned a different value later. TypeScript checks these rules, and the declarations also have corresponding JavaScript binding behavior; they do not provide a special object-freezing feature. See the TypeScript Handbook’s variable declaration guide and its introduction to TypeScript’s static typechecking.
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const taxRate = 0.08;
// taxRate = 0.1; // Error: cannot reassign a const binding
let total = 0;
total += 12; // Allowed: let can be reassignedWhen should you use each one?
Use const when the binding does not need reassignment
The TypeScript Handbook recommends that declarations other than those you plan to modify use const. This is guidance, not a syntax requirement: it makes it easier to see which names remain bound to their initial values and can make data flow more predictable.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchUse let when reassignment is intentional
Choose let when your logic needs to assign a new value to the same variable, such as updating a running total or tracking a changing state. If reassignment is not part of the intended logic, prefer const.
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Do they have different scopes?
No. Both let and const are block scoped: a name declared inside a block, such as an if block, is not available outside that block. Scope is not a reason to choose one over the other. The TypeScript Handbook contrasts them with var, which is function scoped.
Does const make an object or array immutable?
No. const fixes the binding, not every value reachable through it. You can change mutable properties of an object or contents of an array, but you cannot reassign the variable to refer to a different object or array.
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const settings = { theme: "light" };
settings.theme = "dark"; // Allowed: the binding is unchanged
// settings = { theme: "dark" }; // Error: rebinding the constHow are readonly and as const different?
Use readonly in a type when you want TypeScript to disallow assignments to particular properties or array elements through that type. A const declaration controls whether the variable binding can be reassigned; it does not make the properties readonly.
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interface Point {
readonly x: number;
readonly y: number;
}
const point: Point = { x: 1, y: 2 };
// point.x = 3; // Error: x is readonlyThe Handbook puts the distinction succinctly: “Variables use const whereas properties use readonly.” Read more in the TypeScript Handbook’s interfaces guide.
as const is different again: it is a const assertion on a literal expression, not a variable declaration. It preserves literal types rather than widening them, and gives object-literal properties readonly types and array literals readonly tuple types.
const palette = { primary: "blue" } as const;
// primary has the literal type "blue" and is readonlyThese are TypeScript type-level constraints, not runtime freezing. A const assertion also does not make referenced mutable values deeply immutable. The behavior and its limitation are described in the TypeScript 3.4 release notes.
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