TypeScript adds optional static types to JavaScript. A Type in TypeScript means a reference to the value and all its methods and properties. And a Value in JavaScript (and TypeScript) is anything that can be assigned to a variable. That includes String, Boolean, Array, and so on. There are two value categories:

• Primitives: Boolean, Number, String, Symbol, null
• Objects: Array, Object, Function, Class

If we take for example a String data type, it includes many methods and properties, such as length, toUpperCase(), indexOf(), split() and many more. And if we create value by assigning that string to a variable, we will get a Type that will reference to all properties and methods of the string.

All values have a type:

"red car" // a type of String
42        // a type of Number
["one"]   // a type of Array
true      // a type of Boolean

interface Car {
  color: string;
  year: number;
}

{ color: "red", year: 1973 } // an object, a type of Car

Types are essential to TypeScript because the compiler is using them to search and analyses for possible bugs and errors. If you have defined a type of Car with properties color and year, the compiler will make sure that only these properties will exist in a value that you assign the Car type.

Type Annotations

TypeScript also uses types to annotate values. If we create a value of Date and hover over that value, the code editor will show you what type does this value belong to:

Also, it recognizes all the methods and properties this type has. When referencing this value with a dot, and autocomplete will list you all the properties this type has:

And will notify if you try to access undefined methods for this type. It is the best part of Typescript. You will be notified before running your code:

Another example of type annotation when values do not have a predefined type name, just a plain object. Then by default the compiler list all the methods in that object with values and not a short name:

Summing up

TypeScript is based on types. Everything you write, you'll be using types, and a compiler will make sure you won't leave errors and get pretty and helpful annotations.