The process of finding an element in an array using Typescript type guards necessitates first

After creating a type guard to verify if an object is of a specific type, I encountered a type error when trying to use array find with the type guard and a second condition. Strangely, the error disappears when I use array find with the type guard alone followed by find.

Here is the type guard:

const isHtmlField = (
    field: GravityForms.Field
): field is GravityForms.HtmlField => {
    return field.type === 'html'
}

The code snippet below triggers a type error:

const description: GravityForms.HtmlField | undefined = fields.find(
    (item) => {
        isHtmlField(item) && item.label.toLowerCase() === "description";
    }
);

However, the following code works without any issues:

const description: GravityForms.HtmlField | undefined = fields
    .filter(isHtmlField)
    .find((item) => item.label.toLowerCase() === "description");

In my types.d.ts file, the type definitions are as follows:

declare namespace GravityForms {
    // Type definitions here
}

interface FormPage {
    // FormPage interface definition here
}

type FormPages = FormPage[];

type UpdateDataItem = (id: string, value: string) => void;

interface FormSubmissionData {
    // FormSubmissionData interface definition here
}

The complete page component where I'm trying to use the description field can be seen below:

const Page: FC<IPage> = ({ title, fields, hidden, incrementPage }) => {
    // Component code here
};

Answer №1

Your code has a couple of issues that need addressing. Initially:

const description: GravityForms.HtmlField | undefined = fields.find(
    (item) => {
        isHtmlField(item) && item.label.toLowerCase() === "description";
    }
);

The callback function in this section is not acting as a predicate (a function that returns a boolean). Instead, it always returns undefined. To rectify this, you need to replace the curly brackets with round brackets and remove the semicolon for it to function correctly:

const description: GravityForms.HtmlField | undefined = fields.find(
    (item) => (
        isHtmlField(item) && item.label.toLowerCase() === "description"
    )
);

Another issue arises when your callback function uses a type predicate within its body without properly conveying to TypeScript the resultant type of the callback function itself. It would be best to explicitly define the callback function as a type guard:

const descriptionBad: GravityForms.HtmlField | undefined = fields.find(
    (item): item is GravityForms.HtmlField => (
        isHtmlField(item) && item.label.toLowerCase() === "description"
    )
);

Link to playground

That explains why your second function using filter behaves as intended. It utilizes the type guard as the callback function in that instance.

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