Learn React with TypeScript 3

Type annotations

Types for JavaScript variables are determined at runtime. Types for JavaScript variables can also change at runtime. For example, a variable that holds a number can later be replaced by a string. Usually, this is unwanted behavior and can result in a bug in our app.

TypeScript annotations let us declare variables with specific types when we are writing our code. This allows the TypeScript compiler to check that the code adheres to these types before the code executes at runtime. In short, type annotations allow TypeScript to catch bugs where our code is using the wrong type much earlier than we would if we were writing our code in JavaScript.

TypeScript annotations let us declare variables with types using the :Type syntax. ...