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Variables

Q: What is the difference between let and const?

Answer: A variable declared with 'let' can be reassigned to a new value later in the program. A variable declared with 'const' is a 'constant' and its value cannot be changed after it is first assigned..

Q: Can a variable name start with a number?

Answer: No. In JavaScript, variable names (identifiers) must begin with a letter, a dollar sign ($), or an underscore (_). They cannot start with a digit..

Q: Is 'myVariable' the same as 'myvariable' in JS?

Answer: No, JavaScript is case-sensitive. These two names represent two completely different variables..

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Expert Answer & Key Takeaways

Mastering Variables is essential for high-fidelity technical architecture and senior engineering roles in 2026.

JavaScript Variables

Variables are containers for storing data. In JavaScript, there are 4 ways to declare a variable: var, let, const, and by using nothing at all.

1. Variables as Containers

Think of variables as containers that hold values. In the example below, x, y, and z are variables, declared with the let keyword:

let x = 5;
let y = 6;
let z = x + y;

2. The 4 Ways to Declare Variables

Using nothing: (Global by default, not recommended).
Using var: The old way (pre-2015). Avoid unless supporting legacy browsers.
Using let: The modern way to declare variables that can change.
Using const: The modern way to declare variables that cannot change.

3. When to Use `const`?

If you want a general rule: always declare variables with const. If you think the value of the variable can change, use let.

const price1 = 5;
const price2 = 6;
let total = price1 + price2;

4. JavaScript Identifiers (Names)

All JavaScript variables must be identified with unique names. These unique names are called identifiers.

General rules for variable names:

Names can contain letters, digits, underscores, and dollar signs.
Names must begin with a letter.
Names can also begin with $ and _ (but we will not use it in this tutorial).
Names are case sensitive (y and Y are different variables).
Reserved words (like JavaScript keywords) cannot be used as names.

5. The Assignment Operator

In JavaScript, the equal sign (=) is an "assignment" operator, not an "equal to" operator. This is different from algebra.

// In JS, this means: assign the value 10 to x
x = 10;

// This is NOT possible in algebra, but valid in JS:
x = x + 5;

[!NOTE] The "equal to" operator is written like == in JavaScript.

Variables

Expert Answer & Key Takeaways

JavaScript Variables

1. Variables as Containers

2. The 4 Ways to Declare Variables

3. When to Use `const`?

4. JavaScript Identifiers (Names)

General rules for variable names:

5. The Assignment Operator

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Variables

Expert Answer & Key Takeaways

JavaScript Variables

1. Variables as Containers

2. The 4 Ways to Declare Variables

3. When to Use const?

4. JavaScript Identifiers (Names)

General rules for variable names:

5. The Assignment Operator

Top Interview Questions

?Interview Question

?Interview Question

?Interview Question

Course4All Engineering Team

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3. When to Use `const`?