ERROR in TypeError: Cannot read property 'flags' of undefined

I have a problem when I try to run ng serve on a recently created Angular project. The version I'm using is 9.1.4, this error does not occur on a project with an older Angular version.

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.901.4
@angular-devkit/build-angular     0.901.4
@angular-devkit/build-optimizer   0.901.4
@angular-devkit/build-webpack     0.901.4
@angular-devkit/core              9.1.4
@angular-devkit/schematics        9.1.4
@angular/cli                      9.1.4
@ngtools/webpack                  9.1.4
@schematics/angular               9.1.4
@schematics/update                0.901.4
rxjs                              6.5.5
typescript                        3.8.3
webpack                           4.42.0

After executing ng serve or ng build:

chunk {main} main.js, main.js.map (main) 2 kB [initial] [rendered]<br>
chunk {polyfills} polyfills.js, polyfills.js.map (polyfills) 679 bytes [initial] [rendered]<br>
chunk {runtime} runtime.js, runtime.js.map (runtime) 6.15 kB [entry] [rendered]<br>
chunk {styles} styles.js, styles.js.map (styles) 12.4 kB [initial] [rendered]<br>
chunk {vendor} vendor.js, vendor.js.map (vendor) 339 kB [initial] [rendered]<br>
Date: 2020-05-08T02:06:46.145Z - Hash: 1a1b937f0ca2bb37634b - Time: 4087ms<br>

**ERROR in TypeError: Cannot read property 'flags' of undefined**
    at resolveAlias (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:36513:37)
    at checkAliasSymbol (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:64502:26)
    at checkImportBinding (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:64534:13)
    at checkImportDeclaration (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:64552:29)
    at checkSourceElementWorker (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:64961:28)
    at checkSourceElement (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:64800:17)
    at Object.forEach (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:317:30)
    at checkSourceFileWorker (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:65123:20)
    at checkSourceFile (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:65091:13)
    at getDiagnosticsWorker (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:65179:17)
    at Object.getDiagnostics (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:65165:24)
    at C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:98703:85
    at runWithCancellationToken (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:98665:24)
    at getBindAndCheckDiagnosticsForFileNoCache (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:98691:20)
    at getAndCacheDiagnostics (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:98956:26)
    at getBindAndCheckDiagnosticsForFile (C:\Cursos\Angular\ang-hello-world\node_modules\typescript\lib\typescript.js:98688:20)

** **Angular Live Development Server is listening on localhost:4200, open your browser on http://localhost:4200/** **

The result I get on my browser after running this project is:

Cannot GET /

edit

If you use angular 9.1.6 it should be fine now

npm update @angular/cli @angular/core

original post

Something is broken in Angular 9.1.5 - if you run the following it should work:

npm install @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected]

ng build

Just spent about 4-5 hours trying to fix it myself...

The issue seems to stem from using Angular CLI 9.1.4 with Angular 9.1.5 (most likely the compiler

You'll see the issue when you do

ng --version

if you get:

Angular CLI: 9.1.4
Node: 12.16.3
OS: win32 x64

Angular: 9.1.5
... animations, cli, common, compiler, compiler-cli, core, forms
... language-service, platform-browser, platform-browser-dynamic
... router
Ivy Workspace: Yes

Then you will need to follow the npm installs i listed above

9.1.6 just released, should fix that issue

Got the same problem updating from 10.1.0 to 11.1.1.

Updating TypeScript to the latest Version from 4.0.2 to 4.1.3 solved the problem:

npm i typescript@latest

@Matt's answer worked for me as well for ng serve, but needed additional steps. First do these steps "Quoting the steps from his answer":

First check ng --version. Go to next step if you get:

Angular CLI: 9.1.4
Node: 12.16.3
OS: win32 x64

Angular: 9.1.5
... animations, cli, common, compiler, compiler-cli, core, forms
... language-service, platform-browser, platform-browser-dynamic
... router
Ivy Workspace: Yes

Force reinstall the angular packages with 9.1.4 as the version using:

npm install @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected] @angular/[email protected]

Couple of more steps for more ng serve to finally work.

Next error I got was about tslib: An unhandled exception occurred: Cannot find module 'tslib' For this I installed the tslib package manually using npm.
Next error I got was You seem to not be depending on "@angular/core" and/or "rxjs". This is an error.. For this I referred "You seem to not be depending on "@angular/core"." and voila! ng serve worked!

ERROR in TypeError: Cannot read property 'flags' of undefined

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