All language subtitles for 007 Using Union Types_en

Now type inference is great.

And in general, up to this point

we already learned quite a bit

about TypeScript and its core types.

But up to this point, we also always had the scenario

that every variable took just one type.

So we always had just one possible type that could be stored

in a variable,

just a number or just a string or just a boolean.

And often that is what you need,

but sometimes you want to allow multiple, different types.

Let's say here for the course

we might be fine with storing a string in there

but maybe storing a course ID

as a number should also be correct.

It really depends on what you're building.

It's not unrealistic that we could have a variable

where both a string, as well as a number should be allowed

or that you have some variable

which should allow two different kinds of objects.

And for this scenario, that you have more than one type

which you want to allow

you have a feature called union types.

A union type is a type definition that allows more

than one type.

You add it like any other type assignment by adding a colon

after the variable name,

on the left side of the equal sign,

and then your type.

Now I mentioned that explicitly setting string

again here would be redundant, but it's not redundant

if you don't want to use the type inference here.

But if you instead want to specify such a union type,

hence allowing multiple types, you set up such a union type

by adding a pipe symbol after your first type

and then adding another type, like number.

And you can have as many types

which you want to allow here as needed,

but here I just want to have string or number.

And now you see that that error

which we had down here before is gone,

because now we are allowing a number alternatively

to a string value.

And of course you can use such a union type anywhere

where you have a type assignment,

not just when you're using type inference.

I could also use a union type up here

allowing username to be a string or

let's say an array of strings, for example

so that we have first and last name

in an array, for example.

That's also something I could allow here.

Union types are a never key TypeScript feature

which allows you to define more flexible values and types.