Fixed local configuration
Keep a binding from being reassigned after setup.
JS Puzzles · Const
Block Scope
const creates a block-scoped binding that must be initialized immediately and cannot be reassigned. The referenced object or array may still be mutated.
- Since
- ES2015
How it works
A const declaration creates a binding available only within its containing block, function, or module. The binding must be initialized when declared, cannot be reassigned, and cannot be accessed before its declaration is reached because it is in the temporal dead zone. If it refers to an object or array, the binding stays fixed while the referenced contents may still change.
Syntax
const name = value;Key points
- A
constbinding is local to its surrounding block, function, or module rather than being generally visible throughout the program. - Assignment cannot replace the value held by a
constbinding after initialization. - Object and array contents remain mutable unless the referenced value is separately protected.
- A top-level script
constdoes not become a property of the global object. constmust appear in a declaration form; it cannot be used as a standalone statement body.
Common mistakes
constprevents reassignment of the binding, not mutation of an object or array referenced by that binding.- Reading a
constbefore its declaration throws aReferenceError; moving the declaration later does not make the earlier read valid. - A second declaration using the same name in one scope is an error, although a nested block may declare a shadowing binding.
Example: A block-local const shadows an outer binding
const name = "outer";
{
const name = "inner";
console.log(name); // inner
}
console.log(name); // outerThe inner declaration is limited to the block, so it shadows the outer binding only inside those braces.
When to use it
Block-local variables
Limit temporary names to a branch or loop body.
Mutable object state
Share a stable reference while updating object contents.
Interview questions
QHow does const differ from var regarding block scope?
const is block-scoped, so a binding declared inside braces is unavailable outside them. A nested declaration with the same name is a separate binding and can shadow an outer one.
Follow-up: What happens with var in the same block? var is not block-scoped; it belongs to the containing function or script instead.
QWhy must a const declaration be initialized immediately?
A const binding must receive an initializer in its declaration. It cannot later be assigned a value, because reassignment of the binding is disallowed.
Follow-up: Can const x; be made valid? No; unlike an uninitialized let, a const declaration without an initializer is invalid.
QWhat is the temporal dead zone for const?
The declaration is in the temporal dead zone from the beginning of its scope until initialization is processed. Accessing it during that interval throws a ReferenceError.
Follow-up: Does placing the declaration later permit an earlier read? No; the binding exists for scope analysis, but it is inaccessible until execution reaches initialization.
Related
letvarblock statementtemporal dead zoneglobalThis