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How var, let and const differ
The practical differences are scope, what happens if you access a binding before its declaration is reached, whether you can reassign it, and whether you can declare the same name again in the same scope.
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| Keyword | Scope | Access before initialization | Reassignment | Redeclaration in the same scope |
|---|---|---|---|---|
var |
Function scope, or global scope outside a function | Returns undefined before the initializer runs |
Allowed | Allowed |
let |
Block scope | Throws ReferenceError in the temporal dead zone |
Allowed | Not allowed; it is a syntax error |
const |
Block scope | Throws ReferenceError in the temporal dead zone |
Not allowed | Not allowed; it is a syntax error |
What scope means for each keyword
var ignores block boundaries
A var declared inside a function belongs to that function, not just to the nearest pair of braces. If declared outside a function, it has global scope. For example, after this block runs, count is still accessible:
if (true) {
var count = 1;
}
console.log(count); // 1let and const stay inside the block
A block is code enclosed in braces, such as an if body or loop body. A let or const declared there is available only within that block:
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if (true) {
let message = "inside";
const limit = 3;
}
// message and limit are not available hereWhat happens before a declaration is reached
var is declared before execution, but its assignment is not
A var binding is processed before code in its scope executes. Its initializer still runs at the line where it appears, so reading the name earlier produces undefined, not the later assigned value:
console.log(score); // undefined
var score = 10;let and const have a temporal dead zone
A lexical binding declared with let or const exists in its scope before the declaration line, but code cannot access it until execution reaches and initializes that declaration. The intervening period is called the temporal dead zone (TDZ). Access during it throws a ReferenceError:
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console.log(score); // ReferenceError
let score = 10;Even typeof throws if the name is a lexical binding currently in the TDZ. MDN defines the TDZ as the interval “from the start of the block until code execution reaches the place where the variable is declared and initialized.” See MDN’s let reference. Whether to describe let and const as “hoisted” is debated; the TDZ is the behavior to understand.
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Reassignment, redeclaration and const objects
Use let when a binding needs a new value
let allows reassignment after declaration, but you cannot declare that same name again in the same scope:
let status = "pending";
status = "complete"; // allowed
// let status = "failed"; // SyntaxError in the same scopeconst prevents rebinding, not mutation
A const declaration must include an initializer, and the identifier cannot later be assigned a different value. If it refers to an object or array, however, the object’s properties or contents can still change:
const settings = { theme: "light" };
settings.theme = "dark"; // allowed
// settings = {}; // TypeError: cannot reassignThe useful distinction is whether the binding changes, not whether the referenced data can change. Prefer const when the binding remains assigned to the same value; choose let when it must point to a different value.
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Redeclaring names and global declarations
var permits duplicate declarations of the same name in the same scope. let and const do not: attempting to redeclare either in that scope is a syntax error. Reassigning an existing let is different from redeclaring it.
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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 matchIn a classic script, a top-level var declaration creates a property on the global object. Top-level let and const do not. JavaScript modules instead use module scope, so “global” behavior depends on whether the code is a classic script or a module.
Quick Recap
A practical choice for new code
- Start with
constwhen you do not need to assign a different value to the name. - Use
letwhen the name needs reassignment, such as a counter that changes in a loop. - Use
varwhen working with code that already relies on its function-scoped or redeclaration behavior; do not assume it behaves likelet.
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