TypeScript · Const

const

TypeScript’s const creates a block-scoped binding that must be initialized and cannot be reassigned. The referenced object or array may still be mutable unless additional type-level or runtime measures are used.

How it works

Use const for a binding that should not be reassigned. It shares let's block scoping, requires initialization, and does not make an object or array deeply immutable; use readonly or a const assertion when type-level immutability or literal precision is needed.

Syntax

const variableName = initializer;

Key points

const is a variable-declaration form in TypeScript and JavaScript.
After initialization, the const binding cannot be assigned a different value.
const is block-scoped in the same way as let.
Prefer const for declarations whose bindings are not intended to change.

Common mistakes

const prevents rebinding, not mutation of the referenced object or array.

Example: A const binding protects the reference

const settings = { retries: 3 };
settings.retries = 4;

// settings = {}; // Error: cannot reassign a const binding
console.log(settings.retries);

The binding cannot be reassigned, but the referenced object can still be mutated.

When to use it

Stable bindings

A value such as a configuration reference, service instance, or imported dependency should not be rebound after setup.

Default local declarations

A declaration does not need to change during its scope, making least-privilege intent explicit.

Mutable referents

An object or array remains intentionally mutable while its reference stays stable.

Interview questions

QWhat does const guarantee in TypeScript?

const creates a block-scoped binding that must be initialized and cannot later be assigned a new value. It does not freeze the value held by that binding.

Follow-up: How would you preserve literal types in an object? Use an as const assertion, which also makes object properties readonly.

QWhen should a codebase prefer const over let?

Use const when a declaration will not be reassigned; use let when its binding must change. Both follow block-scoping rules, unlike var's function-scoped behavior.

Follow-up: Does const make an object deeply immutable? No; use readonly types or a const assertion for type-level restrictions.

QWhat happens if a const declaration is reassigned?

A const declaration must receive its initial value at the declaration site, and assigning to its binding afterward is an error. The binding can still refer to mutable state.

Follow-up: What does as const change? It prevents literal widening and gives object properties readonly types while producing readonly tuples for arrays.

let
var
readonly
as const

Source: Variable Declaration. Checked 2026-10-15.