External module option

function divide(numerator: number, denominator: number) => Option<number> {
  return denominator == 0.0
   ? None()
   : Some(numerator / denominator);
}

// The return value of the function is an option
const result = divide(2.0, 3.0);

// Pattern match to retrieve the value
result.match({
  // The division was valid
  Some: x => console.log('Result: ', x),
  // The division was invalid
  None: () => console.log('Cannot divide by 0'),
});

Examples

Basic pattern matching on Option:

const msg = Some('howdy');

// Get the value contained by option
const unwrappedMsg = msg.unwrapOr('default message');

Initialize a result to None before a loop:

type Kingdom = {type: 'Plant'|'Animal', size: number, name: string};

const allTheBigThings: [Kingdom] = [
  {type: 'Plant', size: 250, name: 'redwood'},
  {type: 'Plant', size: 230, name: 'noble fir'},
  {type: 'Plant', size: 229, name: 'sugar pine'},
  {type: 'Animal', size: 25, name: 'blue whale'},
  {type: 'Animal', size: 19, name: 'fin whale'},
  {type: 'Animal', size: 15, name: 'north pacific right whale'}
];

// We're going to search for the name of the biggest animal,
// but to start with we've just got `None`.
let nameOfBiggestAnimal: Option<string> = None();
let sizeOfBiggestAnimal = 0;

for (const {type, size, name} of allTheBigThings) {
   if (type === 'Plant') {
     continue;
   }

   if (size > sizeOfBiggestAnimal) {
     sizeOfBiggestAnimal = size;
     nameOfBiggestAnimal = Some(name);
   }
}

nameOfBiggestAnimal.match({
  Some: name => console.log(`the biggest animal is ${name}`);
  None: () => console.log('there are no animals :(');
});