Insight into Typescript types through dot notation

Here is the interface I am working with:

export interface IRenderStruct {
  type: string;
  props: {
    className?: string;
    children?: (string | IRenderStruct) | (string | IRenderStruct)[];
    [propName: string]: any;
  };
}

The objects created based on this interface can have nested structures through elm.props.children. While writing unit tests, I encountered a scenario where I am certain that a RenderStruct object will be the child of another object. There will not be a string or an array of RenderStruct objects as children.

However, when attempting to access the type property like so:

expect(result.props.children.type === 'something');

The TypeScript compiler raises an error stating that I cannot access the type property because children may be a string which does not have the type property.

I am aware of the structure in this particular case. How can I explicitly inform TypeScript about this?

typescript

Answer №1

Implement a custom type guard to verify it:

var display: IDisplayStructure;

// Define a function to ascertain if an object is of type IDisplayStructure:
function isIDisplayStruct(obj: any): obj is IDisplayStructure {
    return ('props' in obj) && ('type' in obj);
}

let isValid: boolean;
if (isIDisplayStruct(display.props.children)) {
    isValid = display.props.children.type === 'something'; // No issues
}

UPDATE:

Specifically for your scenario, you could try this approach:

let testResult = false;
if (isIDisplayStruct(display.props.children)) {
    testResult = display.props.children.type === 'something'; // No errors
}

expect(testResult).toBeTruthy();

This way, you eliminate the need for else statements. If the guard fails, testResult will be false and your test will fail. This method allows for a more comprehensive error handling.

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