Number, String, Boolean
Number Type
The number type is used for both integer and decimal values, since TypeScript, following JavaScript, has just one numeric type instead of separate int and float types. This means 42 and 3.14 are both perfectly valid number values with no special casting needed.
Example: Number Type
let age: number = 42;
let price: number = 3.14;
console.log(age, price);String Type
The string type is used for text values, and TypeScript accepts strings written with single quotes, double quotes, or backtick template literals interchangeably. The type itself doesn't care which quoting style you use, only that the value is text.
Example: String Type
let a: string = 'single';
let b: string = "double";
let c: string = `backtick`;
console.log(a, b, c);Boolean Type
The boolean type represents exactly one of two values, true or false, and is most commonly used for conditions, flags, and yes-or-no states. TypeScript will reject anything else, like the string "true", from being assigned to a boolean-typed variable.
Example: Boolean Type
let isActive: boolean = true;
// isActive = "true"; // rejected: string is not boolean
console.log(isActive);Type Checking
TypeScript checks every assigned value against its declared type at compile time, so assigning an incompatible value, like a string to a number-typed variable, produces an error before the code runs. This catches an entire category of bugs that plain JavaScript would only surface at runtime, if at all.
Example: Type Checking
let count: number = 5;
// count = "5"; // compile-time error: string assigned to number
console.log(count);Using Basic Types Together
Number, string, and boolean types are frequently combined to describe simple program data, such as a user record with an id (number), a name (string), and an isActive flag (boolean). Explicit annotations on each field make the intended shape of that data unmistakable to anyone reading the code.
Example: Using Basic Types Together
interface User {
id: number;
name: string;
isActive: boolean;
}
const user: User = { id: 1, name: "Zoya", isActive: true };
console.log(user);Chapter Quiz — Complete all 11 topics to unlock
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