Union Types and Type Aliases in TypeScript
Sometimes we want our function arguments to be able to accept more than 1 type; e.g. a string or an array. This lesson will show us how to assign more than 1 type to a variable with Typescript union types and type aliases.
Just reporting a typo: let thing = string | number | string[] | boolean; in the transcript should be
type thing = string | number | string[] | boolean; as in the video :)
feel free to use Array.prototype.join instead of forEach ;)
Hi, I think it is now possible to perform some checking with union types and not having typescript yell at us. For example:
type foo = { id: number, name: string };
type bar = { count: number, age: number };
type foobar = foo | bar;
function getFooBar(foobar: foobar) {
if ("id" in foobar) {
// Here's my foo
// foobar.count or foobar.age do not exist
return foobar;
}
if ("count" in foobar) {
// Here's my bar
// foobar.id or foobar.name do not exist
return foobar;
}
}If you paste the previous snippet into the typescript playground source panel and highlight every one of the return statements, you will see that typescript is taking into account that the first object is a foo and that the second is a bar.
I think example in demo.js has. a small mistake in Transcript, i.e., let thing = string | number | string[] | boolean; let returnSomething = (someThing: thing) => { if (typeof someThing === "string" || typeof someThing === "number" || typeof someThing === "boolean") { console.log("somthing = ", someThing); } }
shall be changed to
type thing = string | number | string[] | boolean; let returnSomething = (someThing: thing) => { if (typeof someThing === "string" || typeof someThing === "number" || typeof someThing === "boolean") { console.log("somthing = ", someThing); } }
@sumitj18s Thanks for that! I went ahead and got that fixed in the transcripts. (also noticed the typo 'somthing' and fixed that as well)
Sorry, but that voice is really amazing :)