Type Guard Utility

export function isString(value: unknown): value is string {
  return typeof value === 'string';
}

Type guards in TypeScript help you narrow down the type of a variable, especially when working with values of type unknown or any. This improves type safety and allows for better code suggestions from your editor.

Here's a basic example of a type guard for strings:

export function isString(value: unknown): value is string {
  return typeof value === 'string';
}

How It Works

• The return type value is string is a special TypeScript syntax called a type predicate.
• It allows TypeScript to narrow the type inside a conditional block if the function returns true.

Example Usage

function printLength(val: unknown) {
  if (isString(val)) {
    console.log("Length:", val.length); // TypeScript now knows val is a string
  } else {
    console.log("Not a string.");
  }
}

Why Use Type Guards?

• Prevents runtime errors by ensuring the type before accessing properties.
• Reduces the need for type assertions (as), which can be unsafe.
• Enables safer handling of dynamic data, such as API responses or form inputs.
• Works great in libraries or utility-heavy codebases that handle generic data.

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More Advanced Guards

You can write guards for more complex types, such as custom interfaces or objects.

type User = {
  id: number;
  name: string;
};

export function isUser(value: unknown): value is User {
  return (
    typeof value === 'object' &&
    value !== null &&
    'id' in value &&
    'name' in value
  );
}

function handleUser(input: unknown) {
  if (isUser(input)) {
    console.log(input.name); // Safe access
  } else {
    console.log("Invalid user object.");
  }
}

Bonus: Combine with Type Narrowing

Type guards can be composed with Array.prototype.filter:

const values: unknown[] = ["hello", 42, "world"];

const stringsOnly = values.filter(isString); // string[]

TypeScript now knows stringsOnly is a list of strings thanks to your type guard.

Type guards are a core part of writing safe, scalable TypeScript code — and mastering them gives you confidence when dealing with unknown data.