Convert base64 data-url to javascript File object

b64toFile.ts

// to quickly try the script open 
// https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgJICE4GcIDYAsAInGHMgN4BQyyAtsLRACoCeADhAFzJZhSgBzADTVkMYABsIIOI269+IYaIgAPSCCzAA9iHl9BImgCMCAExJx9igZQC+lSjACuIBGB0hkAfWPY8+EzaAIJQUHAs6M4wMNAAFH44BNaCAJQUogBucFDIxqA5LN4KgsgAvMgk2sYJ-gSpANxZOchSXhX5MlBFJUoAdG0CYAAWTTTZucYskFjlyCAQAO7IAKqgYAAcoeEscW2NojDauXETyMBzAAwN58gAPK3SN8AA1C-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-iwAAMUvAA5OcAHVgBGP9NE8BieRsFi2MPZAAAMABJyCkyw+k-IsIDsPprNs0gO3Uf9PDsczNUU7FVKkQzApQT4vlMvEOOWEK4llL5eMIxUlO451sC4ujjAABRyMARLiuVEr6JSQo0xhtN0zz9N0C0jC+GgxIkg1Cuk8czxQDEvgo9F7EcSg3V4HhZDYKR0AIFYoAkOZzKTOA2GG4AEBITxowAKywXQGkcsoAEYCA2baAGYACYjoAdlwDYAE5Lp9MA2GcMA+jWjbGwIIRNIOrQIG21QEAADRhAApMAAC0-sBla4AAcWU5wzGUoMWAATU0oMJCYWhLv+5TFmAVAVu0YA-uU378YAa3wABZFaVgALwAeUIVAAFZKdphAWHpphAex3GABkSf+oHLmMA6g2GYwwxBiQQYANV+gHUGAGBZcuMpzL6wc8jZXp6QDTjKQCdUlDiLAhpGsaJoOTXNG0KQBm0AQxQCXXiMsA5+rtiAHad178Fd4rLwtA4gA

interface IBase64Data {
  mimeType: string,
  filename: string,
  extension: string,
  b64data: string
}

function _base64ToArrayBuffer(base64: string) {
  var binary_string = atob(base64);
  var len = binary_string.length;
  var bytes = new Uint8Array(len);
  for (var i = 0; i < len; i++) {
    bytes[i] = binary_string.charCodeAt(i);
  }
  return bytes;
}


class Base64String {
  data: IBase64Data
  _b64: string

  constructor(base64String: string) {
    this._b64 = base64String;
    this.data =  this._destructure();
  }

  _destructure() {
    const groups = new RegExp(/(?:data:(.*?);)(?:.*;)*(?:name=(.*)\.(.*?);)(?:.*;)*base64,(.*)/).exec(this._b64);
    const [_, mimeType, filename, extension, b64data] = groups;
    return { mimeType, filename: decodeURIComponent(filename), extension, b64data }
  }

  getBuffer(): Uint8Array {
    return _base64ToArrayBuffer(this.data.b64data)
  }

  getBlob(): Blob {
    return new Blob([this.getBuffer()], { type: this.data.mimeType });
  }

  getFileNameWithextension(): string {
    return `${this.data.filename}.${this.data.extension}`;
  }

  getFile(): File {
    return new File(
      [this.getBlob() as BlobPart],
      this.getFileNameWithextension(),
      { type: this.data.mimeType }
    )
  }
}


const sampleB64Url = `data:application/json;name=1648132276899output.json;base64,W3sie1xcXCJtZXJjaGFudFRyYWRlTm9cXFwiIjoiXFxcIjk4MjUzODI5MzcyOTJcXFwiLFxcXCJ0b3RhbEZlZVxcXCIifV0=`
const base64string = new Base64String(sampleB64Url);

console.log(base64string.data);
console.log(base64string.getFile());