Weeding out Errors Like we mentioned, it’s not unexpected to get error messages after conversion. The important thing is to actually go one by one through these and decide how to deal with the errors. Often these will be legitimate bugs, but sometimes you’ll have to explain what you’re trying to do a little better to TypeScript.

Namespacing As we add more validators, we’re going to want to have some kind of organization scheme so that we can keep track of our types and not worry about name collisions with other objects. Instead of putting lots of different names into the global namespace, let’s wrap up our objects into a namespace. In this example, we’ll move all validator-related entities into a namespace called Validation. Because we want the interfaces and classes here to be visible outside the namespace, we preface

Test the resulting app gulp node dist/main.js The program should print “Hello from TypeScript!”.

Publish to @types Packages on under the @types organization are published automatically from DefinitelyTyped using the types-publisher tool. To get your declarations published as an @types package, please submit a pull request to https://github.com/DefinitelyTyped/DefinitelyTyped. You can find more details in the contribution guidelines page.

Install ASP.NET Core and TypeScript First, install ASP.NET Core if you need it. This quick-start guide uses Visual Studio, which means that you’ll need Visual Studio 2015 in order to use ASP.NET Core. Next, if your version of Visual Studio does not already have TypeScript, you can install it for Visual Studio 2015.

Block-scoping When a variable is declared using let, it uses what some call lexical-scoping or block-scoping. Unlike variables declared with var whose scopes leak out to their containing function, block-scoped variables are not visible outside of their nearest containing block or for-loop. function f(input: boolean) { let a = 100; if (input) { // Still okay to reference 'a' let b = a + 1; return b; } // Error: 'b' doesn't exist here return b; } Here, we have two local v

Test Run the project. You should see a message when you type in the input boxes:

Install typings for dependencies Angular 2 includes es6-shim for Promise support, but TypeScript still needs the types. Open a command prompt, then change directory to the app source: cd C:\Users\<you>\Documents\Visual Studio 2015\Projects\<app>\src\<app> npm install -g typings typings install --global dt~es6-shim

Function declarations Destructuring also works in function declarations. For simple cases this is straightforward: type C = {a: string, b?: number} function f({a, b}: C): void { // ... } But specifying defaults is more common for parameters, and getting defaults right with destructuring can be tricky. First of all, you need to remember to put the type before the default value. function f({a, b} = {a: "", b: 0}): void { // ... } f(); // ok, default to {a: "", b: 0} Then, you need to rememb

Overloads and Callbacks Don’t write separate overloads that differ only on callback arity: /* WRONG */ declare function beforeAll(action: () => void, timeout?: number): void; declare function beforeAll(action: (done: DoneFn) => void, timeout?: number): void; Do write a single overload using the maximum arity: /* OK */ declare function beforeAll(action: (done: DoneFn) => void, timeout?: number): void; Why: It’s always legal for a callback to disregard a parameter, so there’s no need f