How to map over array of multiple types in TypeScript?

Question

If I have a variable whose type is an array that can take multiple types…

type myData = (RedType | BlueType)[];
//`const data` is of type myData

And I am performing a manual check on the types with a conditional to do some logic on the variable data…

     {data.map((element, index) => {
        if (isRed(element)) {
          return <Red data={element} key={index} />;
        } else {
          return <Blue data={element} key={index} />;
        }
      })}

As you can see, I am mapping the array and I return one Component if it is a RedType and I return something else for the BlueType.

However, typescript doesn’t like this, it doesn’t know I am manually doing a conditional. So it errors:

type ‘RedType | BlueType’ is not assignable to type ‘RedType’.

And vise versa.

I will always have an array of multiple types, that I cannot change.

is there no way to do this without having to declare a kind: "red" within the Types? — user10741122
– user10741122, Commented Mar 29, 2019 at 17:08

jcalz · Accepted Answer · 2019-03-29 17:38:54Z

2

You probably want your isRed() function to be a user-defined type guard. That is, change its signature to be something like

declare function isRed(x: any): x is RedType;

and your code should start compiling with no error. Hope that helps; good luck!

answered Mar 29, 2019 at 17:38

jcalz

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jcalz Over a year ago

"it doesn't make" 😕❓

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