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Advanced Generics: Constraints and Defaults

Intermediate
110 XP
45 min
Lesson Content

Advanced Generics

Learn to constrain generics and provide defaults for more powerful type-safe code.

Generic Constraints

interface Lengthwise {
  length: number;
}

function logLength(arg: T): T {
  console.log(arg.length);
  return arg;
}

logLength("hello"); // OK - string has length
logLength([1, 2, 3]); // OK - array has length

Default Type Parameters

function createArray(length: number, value: T): T[] {
  return Array(length).fill(value);
}

Keyof Constraint

function getProperty(obj: T, key: K): T[K] {
  return obj[key];
}
Example Code

Use constraints and keyof

function getValue<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}

const user = { name: "John", age: 30 };
console.log(getValue(user, "name"));
console.log(getValue(user, "age"));

Expected Output:

John
30
Study Tips
  • •Read the theory content thoroughly before practicing
  • •Review the example code to understand key concepts
  • •Proceed to the Practice tab when you're ready to code