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]

Resources

• Basic types