Typescript: Mock a FileList for unit testing purposes.

file-list-helper.ts

interface FileListHelper {
  empty: () => globalThis.FileList,
  fromFiles: (files: File[]) => globalThis.FileList
}

class FileList {
  private readonly files: File[];
  public length: number;
  constructor (files?: File[]) {
    this.files = files || [];
    this.length = files?.length || 0;
  }

  item (index: number) {
    return this.files[index];
  }

  nextFile () {
    let index = -1;
    const files = this.files || [];

    return {
      next: () => ({ value: files[++index], done: !(index in files) })
    };
  }

  [Symbol.iterator] = this.nextFile;
}

export const fileList: FileListHelper = {
  empty: () => {
    const input = document.createElement('input');
    input.type = 'file';
    return input.files;
  },
  fromFiles: (files) => {
    return new Proxy(new FileList(files), {
      get (target, prop, receiver) {
        if (!isNaN(parseInt(String(prop), 10))) {
          return target.item(parseInt(String(prop), 10));
        }
        if (prop !== 'files') return Reflect.get(target, prop, receiver);
      },
      ownKeys (target: FileList): Array<string|symbol> {
        return (['constructor', 'item', 'length', Symbol.iterator]).concat(
          [...Array(target.length).keys()].map(k => String(k))
        );
      },
      getOwnPropertyDescriptor (target, prop) {
        return Object.getOwnPropertyDescriptor(target, prop) || {
          enumerable: true,
          configurable: true
        };
      }
    }) as unknown as globalThis.FileList;
  }
};

usage :

import { fileList } from 'lib/file-list-helper';
const file = new File(['(⌐□_□)'], 'document.pdf', { type: 'application/pdf' });
fileList.fromFiles([file])