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In JavaScript, variables are bindings to values, functions are callable values, and objects group properties behind references. Use const when a binding will not be reassigned, let when it will, and recognize var as older syntax with different scoping rules. Understanding those distinctions helps explain why a block may hide a variable, why a function can remember outer values, and why changing an object through one variable can be visible through another.
How JavaScript variables work
A variable is a named binding that lets code refer to a value. JavaScript is dynamically typed: a binding declared with let can refer to values of different types at different times.
Choose between const and let by reassignment
Use const when you will not assign a different value to the binding. It must be initialized when declared. Use let when the binding needs to receive a new value; an uninitialized let starts as undefined. MDN’s practical guidance is “Use const when you can, and use let when you have to.”
const user = { name: "Ari" };
user.name = "Sam"; // allowed: the binding still refers to the same object
let count = 0;
count += 1; // allowed: count is assigned a new number valueconst prevents reassignment of the binding; it does not freeze the value or recursively freeze data reachable through it. The object example can have its property changed because user still refers to the same object. See MDN’s JavaScript grammar and types guide.
#1 Best Overall
How let, const, and var differ
| Declaration | Scope | Can the binding be reassigned? | Initializer required? | Access before declaration is reached? |
|---|---|---|---|---|
let |
Block scope | Yes | No; without one, its initial value is undefined. |
No. Access during the temporal dead zone throws a ReferenceError. |
const |
Block scope | No | Yes | No. Access during the temporal dead zone throws a ReferenceError. |
var |
Function scope, or global scope when declared outside a function; not block scope | Yes | No | Its behavior differs from let and const; it does not use their temporal dead zone. |
A block is code enclosed in braces, such as an if body. A let or const declared there is available only within that block. A var declared in the same block remains available in its enclosing function or, outside a function, the global scope.
if (true) {
let blockValue = "inside";
var functionValue = "still in the enclosing scope";
}
// blockValue is not available here
console.log(functionValue);JavaScript’s “temporal dead zone” is the period from entering a scope until execution reaches a let or const declaration. Trying to read that binding in the meantime throws a ReferenceError, even if an outer scope has a variable with the same name. MDN documents these declaration and scope rules in its grammar and types guide and JavaScript language overview.
Rank #2
var is legacy syntax that appears in existing code. For new code, block-scoped let and const make the lifetime of a binding easier to see.
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A function is callable code that can accept inputs and return a result. Functions are also values: they can be stored in variables, passed as arguments to other functions, and returned from functions. This lets code package behavior for reuse instead of repeating the same instructions.
function add(a, b) {
return a + b;
}
const operation = add;
console.log(operation(2, 3)); // 5How closures use an enclosing scope
A nested function can access bindings from the function that encloses it. When that inner function continues to use those bindings, this behavior is called a closure.
function makeGreeter(name) {
return function () {
return `Hello, ${name}`;
};
}
const greetAri = makeGreeter("Ari");
console.log(greetAri()); // Hello, AriThe returned function can still read name after makeGreeter has finished. Closures let helper behavior stay near the code that uses it without placing every helper in a wider scope. Scope works in the other direction too: an enclosing function cannot access a variable declared only inside its nested function. For further detail, see MDN’s guide to closures and beginner guide to functions.
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How JavaScript objects and references work
An object groups properties. Read a property with dot notation when its name is a valid identifier, as in user.name, or bracket notation, as in user["name"]. Brackets are useful when the property name is stored in a variable or is not a simple identifier.
const user = { name: "Ari" };
const property = "name";
console.log(user.name); // Ari
console.log(user["name"]); // Ari
console.log(user[property]); // AriAssignments copy object references, not objects
When you assign an object to another binding, JavaScript copies the reference to that object, not a separate object. Both bindings can therefore observe a mutation made through either one.
const first = { score: 1 };
const second = first;
second.score = 2;
console.log(first.score); // 2By contrast, two separately created object literals are distinct objects, even if their properties and values match. The practical distinction is identity: matching contents do not mean two object values are the same object. MDN explains object properties and references in its JavaScript language overview.
Quick Recap
Putting the concepts together
- Use
constfor a binding you will not reassign; useletfor one that needs a new value. - Remember that
constprotects the binding, not the contents of an object it refers to. - Use
letandconstwhen block scope is intended; understand thatvarescapes ordinary block boundaries. - Treat functions as both callable behavior and values that can be passed or returned.
- When two bindings refer to one object, a property mutation through either binding is visible through both.
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