Leveraging File functionality in TypeScript

In the process of developing a web application with Angular 4 and Typescript, I encountered an issue while attempting to retrieve the date of a file for upload. Specifically, when trying to access the lastModified property of a File object, Typescript returned an error:

Property 'lastModified' does not exist on type 'File'.

Upon inspecting the definition, I noticed that it only includes the lastModifiedDate property instead. While this property is marked as deprecated according to https://developer.mozilla.org/en-US/docs/Web/API/File/lastModifiedDate, I found that it functions correctly in Chrome but encounters issues in Safari.

My primary question at this point is: How can I access the File's lastModified property using Typescript?

angulartypescript

Answer №1

Give it a shot

interface CustomFile extends File {
    timestamp: any;
}

let customFile = <CustomFile>originalFile;
let ts = customFile.timestamp;

Answer №2

Also regarding the lastModifiedDate property, although deprecated, it is still present in Chrome browser and used to be the only option in Internet Explorer:

// utilize non-deprecated lastModified field instead 
new Date(file.lastModified)


// applying in-line type assertion
(file as unknown as { lastModifiedDate: Date }).lastModifiedDate

// type assertion with custom type
type DateFile = File & {
  lastModifiedDate: Date;
};
...
(file as DateFile).lastModifiedDate

This will avoid TS2551 error 

Property 'lastModifiedDate' does not exist on type 'File'. Did you mean 'lastModified'? ts(2551)
by using lastModified instead or by asserting its existence.

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