FileReader.onLoad/onLoadEnd event argument has no target.result property

[oriondean]

Trying to retrieve the result of a read file/blob via the event passed into the reader's onload/onloadend method results in a type error (Property 'result' does not exist on type 'EventTarget').

This is valid JavaScript, which is failing type checking because the EventTarget interface does not have a result property.

Examples:

• Typescript playground example - Clicking the run button should show an alert with the file data
• Jsfiddle example

Workaround: Retrieve the result from the reader object (FileReader.result)

[mhegazy]

looks related to #299.

[mhegazy]

PRs welcomed. here is some infromation on contributing lib.d.ts changes: https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes

[JasonGoemaat]

This is used in other cases also such as IndexedDB onsuccess event.

[zhengbli]

Should be covered by microsoft/TypeScript-DOM-lib-generator#207

[victorprocure]

My solution, until this all fixed up, has been just adding a union to the Event callback:

const reader = new FileReader();
reader.onload = (e: Event & { target: { result: string } }) => { ... }

It's gross, but does the trick. The better way would probably be something like this as it is way more complete and abstracts out the types, rather than inlining them.

[LouisWayne]

Instead of using event.target.result, you can just use FileReader.result.

For example,

const fileReader: FileReader = new FileReader();

fileReader.onload = (event: Event) => {
   event.target.result; // This is invalid
   fileReader.result; // This is valid
};