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Functions

Typed parameters and return values, optional and default parameters, and function types.

Runs in Zerethon IDE · TypeScript 7 Official TypeScript documentation ↗
main.ts 
function greet(name: string, greeting = 'Hello'): string {
  return `${greeting}, ${name}!`;
}

function label(name: string, title?: string): string {
  return title ? `${title} ${name}` : name;
}

const apply = (values: number[], fn: (n: number) => number): number[] => values.map(fn);

console.log(greet('Ada'), greet('Linus', 'Hi'));
console.log(label('Ada'), label('Lovelace', 'Countess'));
console.log(apply([1, 2, 3], (n) => n * 10));

Output

Hello, Ada! Hi, Linus!
Ada Countess Lovelace
[ 10, 20, 30 ]
  • Parameters need types (strict mode reports an implicit any); the return type is inferred but writing it documents the function.
  • title?: string is optional — inside the function its type is string | undefined.
  • void is the return type of a function that returns nothing.

Related topics

Official references