Type System Fundamentals

Primitive Types

These are the basic building blocks:

let age: number = 25;
let fullName: string = "Sahil";
let isActive: boolean = true;
let empty: null = null;
let notSet: undefined = undefined;

TypeScript checks if values match your declared type.

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Special Types: any vs unknown

any (unsafe)

let value: any = 10;
value = "text";
value.toUpperCase(); // No error, even if runtime may fail

any disables type safety.

unknown (safe)

let value: unknown = "text";

if (typeof value === "string") {
    value.toUpperCase();
}

unknown forces you to check type before use.

Danger

Overusing any removes the benefits of TypeScript. In interviews and production code, prefer unknown when a value is truly uncertain.

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Annotation vs Inference

Type Annotation

You explicitly write the type. Example: let age: number = 25

Type Inference

TypeScript guesses from value. Example: let age = 25

graph LR A[Variable Declaration] --> B{Type Written?} B -->|Yes| C[Use Annotation] B -->|No| D[Infer From Value]

Practical rule

• Let TypeScript infer local variables.
• Add explicit types on function parameters and public APIs.

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Nullable Types and strictNullChecks

With strict mode, null and undefined are not automatically allowed.

let username: string | null = null;
username = "sahil";

Without | null, assigning null will fail.

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Literal Types

Literal types restrict a value to exact options.

let direction: "left" | "right";
direction = "left"; // valid
// direction = "up"; // error

Great for status values, UI modes, and action names.

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Union Types

let id: string | number;

This means id can be one of the allowed types.

Note

When using a union, only methods common to all members are available unless you narrow first.