Learn React with TypeScript 3

Chapter 5: Advanced Types

1. We have an interface that represents a course result as follows:

interface ICourseMark {  courseName: string;  grade: string;}

We can use this interface as follows:

const geography: ICourseMark = {  courseName: "Geography",  grade: "B"} 

The grades can only be A, B, C, or D. How can we create a stronger typed version of the grade property in this interface?

We can use a union type:

interface ICourseMark {  courseName: string;  grade: "A" | "B" | "C" | "D";}

2. We have the following functions, which validate that numbers and strings are populated with a value:

function isNumberPopulated(field: number): boolean {  return field !== null && field !== undefined;}function isStringPopulated(field: string): boolean { return field ...