Tag: const

JavaScript / ES6+ Bootcamp – Lesson 2

This lesson is important because JavaScript variable scope behaves differently enough from Ruby to cause bugs and int. confusion.

The four things to understand:

let
const
scope
hoisting

let, const, Scope and Hoisting

1. let vs const

Start with the simplest rule:

const name = "Abhilash";
let age = 40;

Use const when the variable should not be reassigned:

const name = "Abhilash";
// name = "John"; ❌

Use let when reassignment is required:

let age = 40;
age = 41; // ✅

Important: const does NOT make the value immutable

This is a very important JavaScript int. question.

const users = [];
users.push("John"); // ✅

The array can be modified.

What cannot happen is:

users = ["Jane"]; // ❌

Why?

Because const prevents reassignment of the variable, not mutation of the object.

Think:

const users
    |
    v
  [ "John" ]

You cannot make users point somewhere else:

users ─────X────> [ "Jane" ]

But you can modify the existing array:

users
  |
  v
["John", "Jane"]

The same applies to objects:

const user = {
  name: "John"
};
user.name = "Jane"; // ✅

But:

user = {}; // ❌

Int. answer

const prevents reassignment, but it does not make objects or arrays immutable.

2. What is scope?

Scope simply means:

Where can this variable be accessed?

Example:

const name = "Abhilash";
function greet() {
  console.log(name);
}
greet();

name is available inside greet() because functions can access variables from their outer scope.

But:

function greet() {
  const message = "Hello";
}
console.log(message); // ❌

message exists only inside the function.

Visualize:

Global Scope
│
├── name
│
└── greet()
     │
     └── message

greet() can see name.

The outside cannot see message.

3. Block scope

This is where let and const become important.

A block is anything inside { }, such as:

if (...)
{
}

or:

for (...)
{
}

Example:

if (true) {
  const message = "Hello";
  console.log(message); // ✅
}
console.log(message); // ❌

message is block scoped.

Same with let:

if (true) {
  let age = 40;
}
console.log(age); // ❌

4. Why var is different

Historically JavaScript used:

var name = "John";

var does not have block scope.

Example:

if (true) {
  var name = "John";
}
console.log(name); // John

That surprises many developers.

Compare:

if (true) {
  let name = "John";
}
console.log(name); // ❌

This is one major reason modern JavaScript prefers let and const.

5. Ruby comparison

You may initially think:

if true
  name = "John"
end
puts name

Ruby’s local-variable behavior is different from JavaScript’s block scoping rules.

For JavaScript, remember:

const + let
      ↓
block scoped

and:

var
 ↓
function scoped

You don’t need to use var in modern code, but you must understand it when reading legacy JavaScript.

6. Nested scope

Scopes can be nested.

const country = "India";
function outer() {
  const state = "Kerala";
  function inner() {
    const city = "Kochi";
    console.log(country);
    console.log(state);
    console.log(city);
  }
  inner();
}

inner() can access:

country
state
city

because it can look outward through its scope chain.

But outer() cannot access city.

Visualize:

Global
│
├── country
│
└── outer
    │
    ├── state
    │
    └── inner
        │
        └── city

JavaScript searches for variables from the current scope outward.

This is called the scope chain.

7. A very important example

Look carefully:

const name = "Abhilash";
function greet() {
  const message = `Hello ${name}`;
  console.log(message);
}
greet();

Inside greet():

name

is not declared locally.

JavaScript looks outward:

greet scope
    ↓
global scope
    ↓
find name

This behavior becomes extremely important when we learn closures.

8. What is hoisting?

Now we get to a classic int. topic.

JavaScript processes declarations before executing the code in the current scope.

This behavior is commonly called hoisting.

But don’t interpret it as JavaScript literally moving your code to the top. That’s a useful mental model, but the actual mechanics are more nuanced.

Start with a function declaration:

greet();
function greet() {
  console.log("Hello");
}

This works.

Why?

Function declarations are available before their textual position.

9. var and hoisting

Consider:

console.log(name);
var name = "John";

You might expect an error.

Instead:

undefined

A useful mental model is:

var name;
console.log(name);
name = "John";

The declaration is available, but the assignment happens later.

10. let and const are different

Now:

console.log(name);
let name = "John";

This produces:

ReferenceError

And:

console.log(name);
const name = "John";

also produces:

ReferenceError

This is often explained using the Temporal Dead Zone (TDZ).

11. Temporal Dead Zone

For let and const, the variable exists in the scope before its declaration is executed, but you cannot access it before that point.

Example:

console.log(age); // ❌
let age = 40;

The area between entering the scope and reaching the declaration is called the:

Temporal Dead Zone

You don’t need to memorize the implementation details yet. The int.-level mental model is:

var
↓
hoisted + initialized as undefined
let / const
↓
hoisted but inaccessible until declaration is reached

12. Function declarations vs function expressions

This becomes important when we get to callbacks.

This works:

greet();
function greet() {
  console.log("Hello");
}

But this does not:

greet();
const greet = function() {
  console.log("Hello");
};

And similarly:

greet();
const greet = () => {
  console.log("Hello");
};

The second forms involve a const variable, so the TDZ applies.

13. A common int. trap

What does this print?

var x = 10;
if (true) {
  var x = 20;
}
console.log(x);

Answer:

20

Because var is function scoped, not block scoped.

Now:

let x = 10;
if (true) {
  let x = 20;
}
console.log(x);

Answer:

10

The inner x belongs to the block.

Visualize:

let x = 10
│
├── if block
│    └── let x = 20
│
└── outside -> x is still 10

14. Shadowing

JavaScript allows an inner scope to define a variable with the same name.

const name = "Abhilash";
function test() {
  const name = "John";
  console.log(name);
}
test();
console.log(name);

Output:

John
Abhilash

The inner variable shadows the outer one.

This is perfectly valid, although unnecessary shadowing can make code harder to read.

15. Why this matters in React

Consider:

function UserList() {
  const [users, setUsers] = useState([]);
  if (users.length === 0) {
    const message = "No users";
    return <p>{message}</p>;
  }
  return (
    <ul>
      ...
    </ul>
  );
}

message exists only inside that if block.

React code frequently contains nested blocks, callbacks and functions, so understanding scope prevents many bugs.

16. Scope + callbacks = important later

Consider:

function createCounter() {
  let count = 0;
  return function() {
    count++;
    console.log(count);
  };
}

Don’t worry about understanding every detail yet.

The interesting question is:

How can the returned function still access count after createCounter() has finished?

That question leads directly to:

closures.

And closures are one of the most important JavaScript concepts for React and Node.js.

Qn) Is it necessary to put semi colon in Javascript?

No, putting a semicolon (or colon) at the end of a line is not mandatory in ES6 (ECMAScript 2015) or modern JavaScript.

JavaScript uses a feature called Automatic Semicolon Insertion (ASI). This means the JavaScript engine automatically inserts semicolons where it thinks they are needed to run your code correctly.

⚠️ The Rare Exception

While you can safely omit semicolons 99% of the time, there are rare cases where leaving them out can break your code. This usually happens if a line starts with a bracket [ or parenthesis (.

Example of a bug without semicolons:

const user = "Alice"
['a', 'b'].forEach(letter => console.log(letter))

The JavaScript engine reads this as a single continuous line: const user = "Alice"['a', 'b'].forEach(...), which throws an error.

Best Practice

Because of ASI, choosing to use semicolons is largely a matter of personal or team preference:

  • With semicolons: Safer for beginners, prevents accidental ASI bugs, and follows traditional coding styles.
  • Without semicolons: Keeps code looking clean, modern, and reduces visual clutter.

Most development teams use a tool like Prettier or ESLint to automatically format the code and handle semicolons for them.

Int. questions

Try answering these without looking back.

Q1

What is the difference between:

const user = {};

and:

let user = {};

Q2

Why does this work?

const users = [];
users.push("John");

even though users is declared with const?

Q3

What is the output?

let x = 10;
if (true) {
  let x = 20;
}
console.log(x);

Q4

What is the output?

var x = 10;
if (true) {
  var x = 20;
}
console.log(x);

Q5

What happens here?

console.log(name);
let name = "John";

Q6

Why does this work?

greet();
function greet() {
  console.log("Hello");
}

but this doesn’t?

greet();
const greet = () => {
  console.log("Hello");
};

Mini exercise

Predict the output before running:

const name = "Abhilash";
function outer() {
  const name = "John";
  if (true) {
    const name = "Jane";
    console.log(name);
  }
  console.log(name);
}
outer();
console.log(name);

Expected mental trace:

global name
    ↓
outer name
    ↓
if-block name

The most important thing from this lesson is to build the habit of asking:

Which scope does this variable belong to?

That question will help enormously when we reach closures, callbacks, useEffect, event handlers, and async JavaScript.

Next lesson: Functions in depth – function declarations, expressions, arrow functions, parameters, return values, callbacks, and higher-order functions.