Sometimes the compiler needs help figuring out a type. In this lesson we learn how to help out the compiler with Typescript type assertion. We have a SuperHero and a BadGuy. Let's make a function that saves the day if the function's argument is a Sup…

It’s easy to pass the wrong value to a function. Typescript interfaces are great because they catch errors at compile time or in an IDE. In this lesson we’ll learn how to describe a type shape with Typescript interfaces. Using interface to describe a…

在 Typescript 2.0 中使用 @type 类型定义 基于 Typescript 开发的时候,很麻烦的一个问题就是类型定义.导致在编译的时候,经常会看到一连串的找不到类型的提示.解决的方式经过了许多的变化,从 DefinitelyTyped 到 typings.最后是 @types.在 Typescript 2.0 之后,推荐使用 @types 方式. DefinitelyTyped 这个工具已经不被推荐,仅作介绍. 多数来自 javascript 的库是没有 TypeScript 类…

Mapped types are a powerful and unique feature of TypeScript's type system. They allow you to create a new type by transforming all properties of an existing type according to a given transformation function. In this lesson, we'll cover mapped types…

The keyof operator produces a union type of all known, public property names of a given type. You can use it together with lookup types (aka indexed access types) to statically model dynamic property access in the type system. Take away: 1. extends 2…

ypeScript 2.2 introduced the object, a type that represents any non-primitive type. It can be used to more accurately type methods such as Object.create. Don't confuse it with the Object type or {}, the empty object type, though! So one thing we need…

TypeScript’s discriminated union types (aka tagged union types) allow you to model a finite set of alternative object shapes in the type system. The compiler helps you introduce fewer bugs by only exposing properties that are known to be safe to acce…

Lookup types, introduced in TypeScript 2.1, allow us to dynamically create types based on the property keys of an object. We'll use the function spyOn from Jest to illustrate how lookup types can type-safe function parameters. Considering this code:…

Conditional types take generics one step further and allow you to test for a specific condition, based on which the final type will be determined. We will look at how they work on a basic level and then leverage their power to allocate function types…

One aspect of control flow based type analysis is that the TypeScript compiler narrows the type of a variable within a type guard. This lesson explores how you can define functions and type predicates to create your own type guards similar to the Arr…