In Typescript you can write type guards to filter down a union type to a single type. For example:
// user-defined type guard
function isFish(pet: Fish | Bird): pet is Fish {
return "swim" in pet;
}
const pet: Fish | Bird = getPet();
// at this point you either have a Fish or Bird
if(isFish(pet)) {
// at this point you (and tsc and intellisense) know you have a Fish
pet.swim();
} else {
// at this point you (and tsc and intellisense) know you have a Bird
pet.fly();
}
JSDoc Type-Checking Version
What if you write your code in Javascript and use JSDoc comments for type-checking and intellisense? You can still write and use type guards!
/** @typedef {{swim: () => void}} Fish */
/** @typedef {{fly: () => void}} Bird */
/**
* @param {Fish | Bird} pet
* @returns {pet is Fish}
*/
function isFish(pet) {
return "swim" in pet;
}
/** @type {Fish | Bird} */
let pet = getPet();
// at this point "pet" is either a Fish or Bird
if (isFish(pet)) {
// at this point you (and tsc and intellisense) know you have a Fish
pet.swim();
} else {
// at this point you (and tsc and intellisense) know you have a Bird
pet.fly();
}
Hopefully you found this post helpful, if you have any questions you can find me on Twitter.
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