How to tell Typescript filtered array of objects contain a key?-CodePudding

In the following code:

interface Obj {
    requiredKey: string;
    optionalKey?: string;
}

type Arr = Array<Obj>;

const arr: Arr = [
    {
        requiredKey: "a",
        optionalKey: "b",
    },
    {
        requiredKey: "c",
    },
];

const filtered = arr.filter((obj) => obj.optionalKey);

How do I tell TS that filtered contains objects where optionalKey exists? Meaning that the type is now:

{
    requiredKey: string;
    optionalKey: string;
}[]

CodePudding user response：

You can use the Required utility type with a user-defined type guard:

interface Obj {
    requiredKey: string;
    optionalKey?: string;
}

const arr: Array<Obj> = [
    {
        requiredKey: "a",
        optionalKey: "b",
    },
    {
        requiredKey: "c",
    },
];

const filtered = arr.filter(
    (obj): obj is Required<Obj> => typeof obj.optionalKey === 'string'
);

CodePudding user response：

We can modify Robby's answer slightly, so that it works without hard-coding the Obj type inside filter and also without accidentally making unrelated properties required.

const filtered = arr.filter(
  (obj): obj is typeof obj & { optionalKey: string } => 
    typeof obj.optionalKey === 'string'
);

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