Dealing with non-existent property in TypeScript while working with JavaScript libraries

Question

Dealing with non-existent property in TypeScript while working with JavaScript libraries

Currently, I'm utilizing TypeScript in a Vue.js application and aiming to implement the drop event to initiate image uploading as shown below:

handleDOMEvents: {
    drop(view, event) {
        const hasFiles = event.dataTransfer
            && event.dataTransfer.files
            && event.dataTransfer.files.length;
        if (hasFiles) {
            upload(event.dataTransfer.files);
        }
    }
}

(For reference, the comprehensive guide I am following can be found here: https://gist.github.com/slava-vishnyakov/16076dff1a77ddaca93c4bccd4ec4521)

The code executes successfully, yet TypeScript raises a warning regarding the .dataTransfer lines:

Property 'dataTransfer' does not exist on type 'Event'

This warning stems from the expectation that the event parameter of the drop function should be an Event object, lacking the dataTransfer property.

Ideally, drop() would always anticipate a DragEvent, but this setting is dictated by an external library beyond my control.

What would be the appropriate course of action in TypeScript? Would simply checking if (typeof(event) !== DragEvent) suffice? Alternatively, how can TypeScript accurately identify this as a DragEvent? Or perhaps, is it prudent to overlook the warning altogether?

I am seeking insight into the best practice for addressing this scenario.

javascript typescript vue.js

Answer 1

Answer №1

While your approach with if (typeof(event) !== DragEvent) is on the right track, it's important to note that TypeScript types do not exist at runtime, so you can't validate them in that manner.

To address this issue, you should consider creating a custom type guard specifically for the event.type field:

function isDragEvent(e: Event): e is DragEvent {
  return e.type === "drag";
}

You can then implement this type guard as a condition within your code:

drop(view, event) {
    if(isDragEvent(event)){
        // TypeScript recognizes event as a DragEvent and allows access to dataTransfer
        const hasFiles = event.dataTransfer
            && event.dataTransfer.files
            && event.dataTransfer.files.length;
        if (hasFiles) {
            upload(event.dataTransfer.files);
        }
    
    } else {
        throw new Error("the passed event is not a DragEvent")
    }
}

Answer 2

While your approach with if (typeof(event) !== DragEvent) is on the right track, it's important to note that TypeScript types do not exist at runtime, so you can't validate them in that manner.

To address this issue, you should consider creating a custom type guard specifically for the event.type field:

function isDragEvent(e: Event): e is DragEvent {
  return e.type === "drag";
}

You can then implement this type guard as a condition within your code:

drop(view, event) {
    if(isDragEvent(event)){
        // TypeScript recognizes event as a DragEvent and allows access to dataTransfer
        const hasFiles = event.dataTransfer
            && event.dataTransfer.files
            && event.dataTransfer.files.length;
        if (hasFiles) {
            upload(event.dataTransfer.files);
        }
    
    } else {
        throw new Error("the passed event is not a DragEvent")
    }
}

Dealing with non-existent property in TypeScript while working with JavaScript libraries

Answer №1

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