Introduction to Type Guards

In TypeScript, a type guard is a way to narrow the type of a value within a specific scope.

Step 1: Define a Type Guard

Create a new file called type-guards.ts and add the following code:

function isString<T>(value: T): value is string {
  return typeof value === 'string';
}

This function is a type guard that checks if a value is a string.

Step 2: Use the Type Guard

Add the following code to the type-guards.ts file:

const value: string | number = 'hello';
if (isString(value)) {
  console.log(value.toUpperCase());
}

This code uses the isString type guard to narrow the type of the value variable.

Step 3: Create a More Complex Type Guard

Add the following code to the type-guards.ts file:

interface Bird {
  fly(): void;
}
interface Fish {
  swim(): void;
}
function isBird<T>(value: T): value is Bird {
  return (value as Bird).fly !== undefined;
}

This code defines a type guard that checks if a value is a Bird.