TypeScript Coach

Types vs data

Types are meta-information - they only exist for the sake of enforcing how actual information is processed. Consider the simple program below.

function isString(str: any): str is string {
    return str != null && typeof str === 'string'
}
let greeting: any = 'hello world'
if (isString(greeting)) {
    const upper: string = greeting.toUpperCase()
    console.log(upper)
}

In the JavaScript produced by the TypeScript compiler, there are no types - only the instructions that operate on data. Attempting to use a type as a value will produce an error.

type IAmType = string
type IAmStillAType = 'string'
function wrong(): string {
    return IAmType
}