TypeScript typeof Operator

The typeof operator in TypeScript is a powerful tool for type checking and type guards. It allows developers to determine the type of a value at runtime, enhancing type safety and enabling more robust code.

Basic Usage

In TypeScript, typeof can be used similarly to JavaScript, but with added type-checking capabilities. It returns a string representing the type of the operand.

let myVariable = "Hello, TypeScript!";
console.log(typeof myVariable); // Output: "string"

let myNumber = 42;
console.log(typeof myNumber); // Output: "number"
    

Type Guards

One of the most powerful features of typeof in TypeScript is its use in type guards. Type guards allow you to narrow down the type of a variable within a conditional block.

function printLength(input: string | number) {
    if (typeof input === "string") {
        console.log(input.length); // TypeScript knows input is a string here
    } else {
        console.log(input.toFixed(2)); // TypeScript knows input is a number here
    }
}
    

Differences from JavaScript

While the typeof operator in TypeScript is similar to its JavaScript counterpart, there are some key differences:

• TypeScript's typeof can be used with custom types and interfaces.
• It provides more precise type information in certain cases.
• It can be used in type contexts, not just value contexts.

Using typeof with Custom Types

TypeScript extends the capabilities of typeof to work with custom types and interfaces:

interface Person {
    name: string;
    age: number;
}

const john: Person = { name: "John", age: 30 };
type PersonType = typeof john; // PersonType is equivalent to the Person interface
    

Best Practices

• Use typeof for runtime type checks and type guards.
• Combine with Union Types for more flexible function parameters.
• Leverage typeof in conditional types for advanced type manipulations.
• Be aware of its limitations with complex objects and custom types.

Related Concepts

To deepen your understanding of TypeScript's type system, explore these related topics:

• Type Inference in TypeScript
• keyof Operator for working with object keys
• Conditional Types for advanced type manipulations

By mastering the typeof operator, you'll enhance your ability to write type-safe and robust TypeScript code. It's an essential tool in the TypeScript developer's toolkit, enabling more precise type checking and improved code quality.