Basic types
Basic (primitive) types in JavaScript are String, Number, Boolean, null, undefined, and Symbol.
Types in TypeScript add couple more: any, void, and never. Let's add some types to the code!
let baz: any = "hello";
baz = 12;
The any type is the default value for any variable or argument: this behaves exactly like JavaScript.
const foo: number = 12;
const bar: boolean = false;
const title: string = "Hello, Devmeetings!";
const onlyThisVerySpecificString: "specific string" = "specific string";
Unions
The last one is quite fun: the variable can only have one particular value, everything else will throw an error. Let's make this useful:
const eventType: "workshop" | "boring" = "workshop";
Now the eventType variable can only have one of the two values. This is called a union type and can be used with all types:
let numberOrString: number | string;
Built-in types
There is some more complex built-in types that we can use:
const start: Date = new Date();
const e: Error = new Error("boo!");
Arrays
It's also possible to tell TypeScript that we expect an array of some elements:
const numbers: number[] = [1, 2, 3]
We can also provide types for individual elements in an array (tuple types):
const tuple: [string, number] = ["hello", 42]
Tuples require an exact number of elements at exact positions.
Custom types
We can declare custom types using the type keyword:
type EventType = "workshop" | "boring";
And then use them in same way as any other type:
const eventType: EventType = "workshop";
const resultTuple: [EventType, number] = ["boring", 1337]