Ambient enums Ambient enums are used to describe the shape of already existing enum types. declare enum Enum { A = 1, B, C = 2 } One important difference between ambient and non-ambient enums is that, in regular enums, members that don’t have an initializer are considered constant members. For non-const ambient enums member that does not have initializer is considered computed.

Array TypeScript, like JavaScript, allows you to work with arrays of values. Array types can be written in one of two ways. In the first, you use the type of the elements followed by [] to denote an array of that element type: let list: number[] = [1, 2, 3]; The second way uses a generic array type, Array<elemType>: let list: Array<number> = [1, 2, 3];

The JSX result type By default the result of a JSX expression is typed as any. You can customize the type by specifying the JSX.Element interface. However, it is not possible to retrieve type information about the element, attributes or children of the JSX from this interface. It is a black box.

Union Types Union types are closely related to intersection types, but they are used very differently. Occasionally, you’ll run into a library that expects a parameter to be either a number or a string. For instance, take the following function: /** * Takes a string and adds "padding" to the left. * If 'padding' is a string, then 'padding' is appended to the left side. * If 'padding' is a number, then that number of spaces is added to the left side. */ function padLeft(value: string, paddin

Red flags

Dependencies on Modules If your library depends on a module, use an import statement: import * as moment from "moment"; function getThing(): moment;

Add TypeScript code Right click on scripts and click New Item. Then choose TypeScript File (it may be in the .NET Core section) and name the file app.ts.

Symbol.hasInstance A method that determines if a constructor object recognizes an object as one of the constructor’s instances. Called by the semantics of the instanceof operator.

Dependencies on Global Libraries If your library depends on a global library, use a /// <reference types="..." /> directive: /// <reference types="someLib" /> function getThing(): someLib.thing;

Void void is a little like the opposite of any: the absence of having any type at all. You may commonly see this as the return type of functions that do not return a value: function warnUser(): void { alert("This is my warning message"); } Declaring variables of type void is not useful because you can only assign undefined or null to them: let unusable: void = undefined;