...
"dependencies": {
  ...
  "@types/filesaver": "0.0.30",
  "file-saver": "^1.3.2"
  ...
}
...

import { saveAs } from 'file-saver';
...
let file = new Blob(['hello world'], { type: 'text/csv;charset=utf-8' });
saveAs(file, 'helloworld.csv');
...

import * as fileSaver from 'file-saver';

npm i -S file-saver

"scripts": ["../node_modules/file-saver/FileSaver.js"]

declare function saveAs();

export class MyClass{
  constructor(){
    saveAs(blablabla...)
  }
}

npm install file-saver --save