Welcome back! I am Mihir, and in this lesson we will learn the boolean type in TypeScript.

A boolean can only be one of two values:

true
false

Booleans are used for conditions, flags, permissions, loading states, feature toggles, and form validation.

What is a boolean?

Use boolean for true or false values.

let isLoggedIn: boolean = true;
let hasAccess: boolean = false;
let isPublished: boolean = true;

Invalid:

let isActive: boolean = "yes";

"yes" is a string, not a boolean.

Type Inference with Booleans

You do not always need : boolean.

let isLoading = false;
let isAdmin = true;

TypeScript infers both as boolean.

Invalid:

isLoading = "no";
isAdmin = 1;

TypeScript already knows these variables should be booleans.

Booleans in Conditions

Booleans are commonly used with if statements.

let isLoggedIn: boolean = true;

if (isLoggedIn) {
  console.log("Show dashboard");
} else {
  console.log("Show login page");
}

This makes your program choose between different paths.

Boolean Function Parameters

You can pass booleans into functions.

function setTheme(isDarkMode: boolean): void {
  if (isDarkMode) {
    console.log("Use dark theme");
  } else {
    console.log("Use light theme");
  }
}

setTheme(true);

Invalid:

setTheme("dark");

The function expects a boolean.

Boolean Function Return Values

Functions can return booleans.

function isAdult(age: number): boolean {
  return age >= 18;
}

const canVote = isAdult(21);

canVote is a boolean.

Another example:

function hasMinLength(value: string, minLength: number): boolean {
  return value.length >= minLength;
}

This is common in form validation.

Booleans in Object Types

Booleans are often used as flags in objects.

type User = {
  id: number;
  name: string;
  isAdmin: boolean;
  emailVerified: boolean;
};

const user: User = {
  id: 1,
  name: "Mihir",
  isAdmin: false,
  emailVerified: true,
};

The names should be clear and readable.

Good boolean names often start with:

• is
• has
• can
• should

Boolean Expressions

Comparison expressions return booleans.

const age = 25;
const isAdult = age >= 18;

TypeScript infers isAdult as boolean.

Other examples:

const isExact = "TypeScript" === "TypeScript";
const hasItems = [1, 2, 3].length > 0;
const isValidPrice = 999 > 0;

Each result is a boolean.

Boolean Literal Types

You can use literal boolean types, but they are less common.

let alwaysTrue: true = true;

Invalid:

alwaysTrue = false;

You will usually use boolean, not true or false alone.

Boolean literal types become more useful in advanced patterns with discriminated unions.

Common Mistake: boolean vs Boolean

Use lowercase boolean:

let active: boolean = true;

Avoid uppercase Boolean:

let active: Boolean = true;

For normal true or false values, lowercase boolean is the correct choice.

Common Mistake: String Booleans

These are strings:

let value = "true";
let enabled = "false";

They are not booleans.

If data comes from a URL, form, or API, convert it intentionally:

function parseBoolean(value: string): boolean {
  return value === "true";
}

Do not assume "false" behaves like false.

In JavaScript, "false" is a truthy string.

Quick Reference Summary

Practice

Create a small validation function:

function canPublish(title: string, isDraft: boolean): boolean {
  return title.trim().length > 0 && !isDraft;
}

const result = canPublish("TypeScript Basics", false);

console.log(result);

Try passing "false" instead of false and see what TypeScript says.

What You've Learned

You now understand:

• What the boolean type is
• How boolean inference works
• How booleans are used in conditions
• How to type boolean parameters and returns
• How booleans work inside object types
• Why clear boolean names matter
• Why string booleans are not real booleans
• Why lowercase boolean is preferred

What's Next?

In the next lesson, we will learn null and undefined in TypeScript.

These two values are important for missing data, loading states, optional fields, and strict null checking.

Need Help?

• Have questions, confusion, or want to know more? Contact me

Booleans are simple, but naming them well makes your code much easier to read.