If Angular is not running in Internet Explorer you may need to uncomment out a chunk of polyfills.ts.

Depending on how old the Angular app is, it may not even have a polyfill.ts, but if it is there looks for something like so to uncomment:

/** IE9, IE10 and IE11 requires all of the following polyfills. **/
// import 'core-js/es6/symbol';
// import 'core-js/es6/object';
// import 'core-js/es6/function';
// import 'core-js/es6/parse-int';
// import 'core-js/es6/parse-float';
// import 'core-js/es6/number';
// import 'core-js/es6/math';
// import 'core-js/es6/string';
// import 'core-js/es6/date';
// import 'core-js/es6/array';
// import 'core-js/es6/regexp';
// import 'core-js/es6/map';
// import 'core-js/es6/weak-map';
// import 'core-js/es6/set';
 
/** IE10 and IE11 requires the following for NgClass support on SVG elements */
// import 'classlist.js'; // Run `npm install --save classlist.js`.
 
/** IE10 and IE11 requires the following for the Reflect API. */
// import 'core-js/es6/reflect';

 
 

You will probably need to update the node_modules folder with "npm install" on the other side of the uncomment out. If you get TLS errors in IE try leading localhost:4200 with https:// instead of http://