Modern JavaScript style guides treat `var` as legacy. Knowing exactly when to reach for `const` versus `let` — and understanding what `const` actually protects — is one of the fastest ways to spot a professional codebase from an amateur one.
1let vs const Best Practices | JavaScript Tutorial - In-Depth Guide Part 1
Professional JavaScript defaults to 'const' for every binding, and only reaches for 'let' when a variable must be reassigned. 'var' is considered legacy.
const MAX_RETRIES = 3;
let attempts = 0;const First
2let vs const Best Practices | JavaScript Tutorial - In-Depth Guide Part 2
'let' and 'const' are block-scoped — confined to the nearest { } — unlike 'var', which leaks out to the whole function.
{
let x = 1;
}
console.log(x); // ReferenceErrorBlock Scope
3let vs const Best Practices | JavaScript Tutorial - In-Depth Guide Part 3
Both 'let' and 'const' live in the Temporal Dead Zone until their declaration line runs — accessing them earlier throws, instead of silently returning undefined like 'var' would.
console.log(y); // ReferenceError
let y = 5;Temporal Dead Zone
4let vs const Best Practices | JavaScript Tutorial - In-Depth Guide Part 4
'const' does not make a value immutable — it only locks the binding. Object and array contents behind a const reference remain fully mutable.
const user = { name: 'Ana' };
user.name = 'Bea'; // OK
user = {}; // TypeErrorconst ≠ Frozen
5let vs const Best Practices | JavaScript Tutorial - In-Depth Guide Part 5
The professional rule of thumb: default to 'const' everywhere, and only drop to 'let' for values that genuinely need reassignment, like loop counters or accumulators.
for (let i = 0; i < 5; i++) {
// fresh i per iteration
}let for Iteration
6Step-by-Step Breakdown
Professional JavaScript defaults to 'const' for every binding, and only reaches for 'let' when a variable must be reassigned. 'var' is considered legacy.
'let' and 'const' are block-scoped — confined to the nearest { } — unlike 'var', which leaks out to the whole function.
Checkpoint: Is a variable declared with let inside a { } block accessible outside of it?
- →Yes, block braces do not affect scope
- →No, it is confined to that block
Both 'let' and 'const' live in the Temporal Dead Zone until their declaration line runs — accessing them earlier throws, instead of silently returning undefined like 'var' would.
'const' does not make a value immutable — it only locks the binding. Object and array contents behind a const reference remain fully mutable.
Checkpoint: Does reassigning a *property* of a const object throw an error?
- →Yes, const locks the entire object
- →No, only rebinding the variable itself throws
The professional rule of thumb: default to 'const' everywhere, and only drop to 'let' for values that genuinely need reassignment, like loop counters or accumulators.
Next, we'll explore 'Destructuring Assignment'.
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Browser Support
Fully supported.
Fully supported.
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Fully supported.
Accessibility (A11y)
1Predictable Scoping Reduces State-Related ARIA Bugs
Widgets that manage focus or aria-expanded state via closures over let/const bindings behave more predictably than var-based code, where a leaked binding can cause one widget instance to silently affect another.
SEO Implications
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Const-by-Default Code Is Easier to Tree-Shake
Bundlers can more confidently eliminate unused const bindings than var ones, since const guarantees the reference never changes — smaller shipped JavaScript bundles improve Largest Contentful Paint and Time to Interactive.
Best Practices
Default Every Declaration to `const`
Starting with const and only relaxing to let when the linter flags a reassignment documents intent for every reader: a const binding is a promise that the reference never changes.
Never Use `var` in New Code
var's function-scoping and hoisting behavior are responsible for entire categories of bugs that block scoping eliminates outright; there is no situation in modern JS where var is the better choice.
Frequent Bugs
A `var` declared inside a `for` loop shares one binding across every iteration, so async callbacks registered inside the loop all log the same final value.
Switch the loop counter to `let`, which creates an independent binding for each iteration, giving every closure its own captured value.
Code assumes `const` prevents an array or object from being mutated, then is surprised when `.push()` or property assignment succeeds.
Remember const only locks the variable binding. Use `Object.freeze()` (shallow) or a deep-freeze utility if true immutability is required.
Real-World Examples
Configuration Objects with const
An app-wide config object needed to be assembled once at startup and never reassigned, while still allowing individual settings to be read throughout the app.
const config = {
apiUrl: 'https://api.example.com',
timeoutMs: 5000,
};
// Later, in any module:
fetch(`${config.apiUrl}/users`, { signal: AbortSignal.timeout(config.timeoutMs) });