devmeetings-react-ts-fork
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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!1
let baz: any = "hello";
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baz = 12;
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The
any type is the default value for any variable or argument: this behaves exactly like JavaScript.1
const foo: number = 12;
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const bar: boolean = false;
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const title: string = "Hello, Devmeetings!";
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const onlyThisVerySpecificString: "specific string" = "specific string";
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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:
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const eventType: "workshop" | "boring" = "workshop";
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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:1
let numberOrString: number | string;
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Built-in types
There is some more complex built-in types that we can use:
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const start: Date = new Date();
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const e: Error = new Error("boo!");
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Arrays
It's also possible to tell TypeScript that we expect an array of some elements:
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const numbers: number[] = [1, 2, 3]
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We can also provide types for individual elements in an array (
tuple types):1
const tuple: [string, number] = ["hello", 42]
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Tuples require an exact number of elements at exact positions.
Custom types
We can declare custom types using the
type keyword:1
type EventType = "workshop" | "boring";
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And then use them in same way as any other type:
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const eventType: EventType = "workshop";
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const resultTuple: [EventType, number] = ["boring", 1337]
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