Read files with FileReader

File.ts

export interface IFile {
  name: string
  size?: number
  type?: string
  lastModified?: number
  data?: string|ArrayBuffer
}

export enum ReadFileReturnTypes {
  Text = "text",
  DataUrl = "dataUrl",
  BinaryString = "binaryString",
  ArrayBuffer = "arrayBuffer"
}

export const readFile = (file: File, returnType: ReadFileReturnTypes = ReadFileReturnTypes.DataUrl): Promise<IFile> =>  {
  const reader = new FileReader()
  return new Promise((resolve, reject) => {
    reader.onload = (event:ProgressEvent<FileReader>) => {      
      const abstractFile = {
        data: event.target.result,
        name: file.name,
        size: file.size,
        type: file.type,
        lastModified: file.lastModified
      }
      resolve(abstractFile)
    }     
    reader.onerror = reject

    switch(returnType) {
      case ReadFileReturnTypes.DataUrl : reader.readAsDataURL(file)
      break
      case ReadFileReturnTypes.BinaryString : reader.readAsBinaryString(file) 
      break
      case ReadFileReturnTypes.ArrayBuffer : reader.readAsArrayBuffer(file) 
      break
      default : reader.readAsText(file)
    }
  })
}

export const readFilesAsync = async (files: FileList, returnType: ReadFileReturnTypes = ReadFileReturnTypes.DataUrl): Promise<IFile[]> => 
  await Promise.all(Array.from(files).map(file => readFile(file, returnType)))