//1 step
const convertBase64ToFile = (base64String, fileName) => {
     let arr = base64String.split(',');
     let mime = arr[0].match(/:(.*?);/)[1];
     let bstr = atob(arr[1]);
     let n = bstr.length;
     let uint8Array = new Uint8Array(n);
     while (n--) {
        uint8Array[n] = bstr.charCodeAt(n);
     }
     let file = new File([uint8Array], fileName, { type: mime });
     return file;
}

//2 step
npm i --save-dev @types/file-saver

//3 step
import { saveAs } from 'file-saver';

//4 step
const downloadBase64Data = (base64String, fileName) => {
     let file = convertBase64ToFile(base64String, fileName);
     saveAs(file, fileName);
}