Functions
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TypeScript Parameter Type Annotations
Function parameters may be given type annotations with the same syntax as variable declarations – a colon next to the name. The type annotations ensure that the parameters are of the correct type.
functiongreet(noun: string){console.log(`Hello,${noun}!`);}greet('World');// Prints: Hello, Worldgreet(2020);// Argument of type 'number' is not assignable to parameter of type 'string'.
TypeScript Optional Parameter
Sometimes we would like to skip providing values to function calls. We can declare some parameters in a function to be optional if a value is not provided for all arguments in a function. We do this by adding a question mark symbol,?, after the parameter name before the colon,:.
functiongreet(name?: string){console.log(`Hello,${ name||'stranger'}!`);}greet();// Prints: Hello, stranger!
TypeScript Default Parameters
If we assign a function parameter to have a default value, TypeScript will infer the parameter type to be the same as the default value’s type.
functionexponentiation(power=1){console.log(4** power);}exponentiation();// Prints: 4exponentiation(4);// Prints: 256exponentiation(true);// Error: Argument of type 'true' is not assignable to parameter of type 'number | undefined'.
TypeScript Inferring Return Types
By looking at the types of the values in a function’s return statement, TypeScript can infer thereturn type of a function.
functionfactOrFiction(){return Math.random()>=.5?'true':'false';}constmyAnswer: boolean=factOrFiction();// Type 'string' is not assignable to type 'boolean'
TypeScript Void Return Type
If a function does not return any value, then you can specifyvoid as a return type using type annotation.
functionsayHello():void{console.log('Hello!')}
TypeScript Explicit Return Types
We can be explicit about what type a function returns by adding type annotation (: followed by the type) after a function’s closing parenthesis,).
functiontrueOrFalse(value: boolean): boolean{if(value){returntrue;}return'false';// Typescript Error: Type 'string' is not assignable to type 'boolean'.}
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- Learn TypeScript, a superset of JavaScript that adds types to make the language scale!
- Intermediate.10 hours