Error: Can't resolve 'core-js/es7/reflect' in '\node_modules\@angular-devkit\build-angular\src\angular-cli-files\models

Question

After updating to Angular 7.3.6, I get the following error on ng serve:

ERROR in ./node_modules/@angular-devkit/build-angular/src/angular-cli-files/models/jit-polyfills.js Module not found: Error: Can't resolve 'core-js/es7/reflect' in '\node_modules@angular-devkit\build-angular\src\angular-cli-files\models'

Here is my package.json dependecies:

{
  "name": "frontend",
  "version": "0.0.0",
  "license": "MIT",
  "scripts": {
    "ng": "ng",
    "start": "ng serve",
    "build": "ng build --prod",
    "test": "ng test",
    "lint": "ng lint",
    "e2e": "ng e2e"
  },
  "private": true,
  "dependencies": {
    "@angular/animations": "^7.2.11",
    "@angular/cdk": "^7.3.6",
    "@angular/common": "^7.2.11",
    "@angular/compiler": "^7.2.11",
    "@angular/core": "^7.2.11",
    "@angular/flex-layout": "^7.0.0-beta.24",
    "@angular/forms": "^7.2.11",
    "@angular/http": "^7.2.11",
    "@angular/material": "^7.3.6",
    "@angular/platform-browser": "^7.2.11",
    "@angular/platform-browser-dynamic": "^7.2.11",
    "@angular/router": "^7.2.11",
    "adm-zip": "^0.4.13",
    "core-js": "^2.6.5",
    "hammerjs": "^2.0.8",
    "rxjs": "^6.4.0",
    "rxjs-compat": "^6.0.0-rc.0",
    "tslib": "^1.9.3",
    "zone.js": "^0.9.0"
  },
  "devDependencies": {
    "@angular-devkit/build-angular": "^0.13.7",
    "@angular/cli": "^7.3.7",
    "@angular/compiler-cli": "^7.2.11",
    "@angular/language-service": "^7.2.11",
    "@types/jasmine": "~3.3.12",
    "@types/jasminewd2": "~2.0.6",
    "@types/node": "~11.12.0",
    "codelyzer": "^5.0.0",
    "jasmine-core": "~3.3.0",
    "jasmine-spec-reporter": "~4.2.1",
    "karma": "^4.0.1",
    "karma-chrome-launcher": "~2.2.0",
    "karma-coverage-istanbul-reporter": "^2.0.5",
    "karma-jasmine": "~2.0.1",
    "karma-jasmine-html-reporter": "^1.4.0",
    "protractor": "^6.0.0",
    "ts-node": "~8.0.3",
    "tslint": "~5.14.0",
    "typescript": "3.1.6"
  }
}

Answer 1

You should install core-js@2.5.x that contains this file/module. See preset-env docs.

Also note, however, that this is a very old version with known issues and is unsupported. From the module itself:

"core-js@<3.23.3 is no longer maintained and not recommended for usage due to the number of issues. Because of the V8 engine whims, feature detection in old core-js versions could cause a slowdown up to 100x even if nothing is polyfilled. Some versions have web compatibility issues. Please, upgrade your dependencies to the actual version of core-js."

Please consider using one of the better solutions below.

Answer 2

I solved the problem by removing the number at the end of "es" in the path as in response to Module not found: Error: Can't resolve 'core-js/es7/reflect

Answer 3

In my case , i just changed -->

import 'core-js/es6/reflect';
import 'core-js/es7/reflect';

to

import 'core-js/es/reflect';

and it worked..

Answer 4

I found possible answer. You have core-js version 3.0, and this version doesn't have separate folders for ES6 and ES7; that's why the application cannot find correct paths.

To resolve this error, you can downgrade the core-js version to 2.5.7. This version produces correct catalogs structure, with separate ES6 and ES7 folders.

To downgrade the version, simply run:

npm i -S core-js@2.5.7

Answer 5

For Angular 7

From package.json remove:

"core-js": "3.6.5",

because it is part of the Angular package.

Answer 6

For angular latest version (currently -> 9) ,

compatible core-js version is "core-js": "~2.5.0".

step 1) Delete Node modules.

step 2) In package.json file, write "core-js": "~2.5.0" in dependecies object.

step 3) npm install

then It will work perfectly fine

Answer 7

Removing the number at the end of 'es' in the path, like 'core-js/es/reflect' worked also for me.

For EX: import 'core-js/es7/reflect'; was changed to import 'core-js/es/reflect';

Answer 8

If you'd like to use version 3.0, you can add the a path to your tsconfig.json file.

{
    "compilerOptions": {
        ...
        "paths": {
            "core-js/es7/reflect": [
                "node_modules/core-js/proposals/reflect-metadata"
            ]
        }
    }
}

Note: You need check the node_modules/core-js/proposals/reflect-metadata relative path and correct it if needed in your project structure.

Answer 9

For Angular 8 (source):

Angular CLI 8.0+ manages the required Angular polyfills directly and projects will not require a direct dependency on core-js (assuming an application does not manually include additional core-js polyfills).

I've resolved by:

• removing core-js from my package.json dependencies
• removing import 'core-js/es7/reflect'; from my test.ts

Answer 10

I fixed it by adding this import: import 'core-js/proposals/reflect-metadata'; in my polyfills.ts file

/** Evergreen browsers require these. **/
// Used for reflect-metadata in JIT. If you use AOT (and only Angular decorators), you can remove.
import 'core-js/proposals/reflect-metadata';

Make sure you have the latest core-js update and everything should work.

Answer 11

The tsconfig.json solution from @medeni works, but don't copy the path before checking it by yourself. For me it had to be:

"paths": {
  "core-js/es7/reflect": ["../node_modules/@angular-devkit/build-angular/node_modules/core-js/proposals/reflect-metadata"]
}

the ../ is because tsconfig.json is extended by src/tsconfig.app.json or src/tsconfig.spec.json, so /src is the working directory.

note: I am using Angular CLI 8.1.0

Answer 12

The version 3.0 of core-js has some breaking changes:

https://github.com/zloirock/core-js/blob/master/docs/2019-03-19-core-js-3-babel-and-a-look-into-the-future.md

You should find a line similar to this in one of your files: import "core-js/es7/reflect";

Change it to this: import "core-js/proposals/reflect-metadata";