npm: When to use `--force` and `--legacy-peer-deps`

Solution 1:

In the new version of npm (v7), by default, npm install will fail when it encounters conflicting peerDependencies. It was not like that before.

Take a look here for more info about peer dependencies in npm v7.

The differences between the two are below -

--legacy-peer-deps: ignore all peerDependencies when installing, in the style of npm version 4 through version 6.
--strict-peer-deps: fail and abort the install process for any conflicting peerDependencies when encountered. By default, npm will only crash for peerDependencies conflicts caused by the direct dependencies of the root project.

Solution 2:

In the article of https://github.blog/2021-02-02-npm-7-is-now-generally-available/

You have the option to retry with --force to bypass the conflict or --legacy-peer-deps command to ignore peer dependencies entirely (this behavior is similar to versions 4-6).

I agree this sentence is not really clear, but "ignore peer dependencies entirely" does not sound good. Let's use a real example:

Here is a peer dependency error I met when I npm install:

npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR!
npm ERR! While resolving: mobile@undefined
npm ERR! Found: [email protected]
npm ERR! node_modules/react
npm ERR!   react@"17.0.1" from the root project
npm ERR!   peer react@">=16.0.0" from @testing-library/[email protected]
npm ERR!   node_modules/@testing-library/react-native
npm ERR!     dev @testing-library/react-native@"7.2.0" from the root project
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! peer react@"16.13.1" from [email protected]
npm ERR! node_modules/react-native
npm ERR!   react-native@"https://github.com/expo/react-native/archive/sdk-39.0.4.tar.gz" from the root project
npm ERR!   peer react-native@">=0.59" from @testing-library/[email protected]
npm ERR!   node_modules/@testing-library/react-native
npm ERR!     dev @testing-library/react-native@"7.2.0" from the root project
npm ERR!
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force, or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.
npm ERR!
npm ERR! See /Users/me/.npm/eresolve-report.txt for a full report.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/me/.npm/_logs/2021-03-13T00_10_33_813Z-debug.log
npm ERR! code 1
npm ERR! path /Users/me/my-app
npm ERR! command failed
npm ERR! command sh -c sh ./bin/setup.sh

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/me/.npm/_logs/2021-03-13T00_10_33_860Z-debug.log

Below is the package-lock.json difference between --legacy-peer-deps and --force.

If I run npm install --legacy-peer-deps, it adds this in my package-lock.json:

"node_modules/@unimodules/react-native-adapter": {
  "version": "5.7.0",
  "resolved": "https://registry.npmjs.org/@unimodules/react-native-adapter/-/react-native-adapter-5.7.0.tgz",
  "integrity": "sha512-L557/+sc8ZKJVgo1734HF1QNCxrt/fpqdmdNgySJT+kErux/AJNfPq3flsK0fyJduVmniTutYIMyW48cFoPKDA==",
  "dependencies": {
    "invariant": "^2.2.4",
    "lodash": "^4.5.0"
  },
  "peerDependencies": {
    "react-native": "*",
    "react-native-web": "~0.13.7"
  }
},

...

"@unimodules/react-native-adapter": {
  "version": "5.7.0",
  "resolved": "https://registry.npmjs.org/@unimodules/react-native-adapter/-/react-native-adapter-5.7.0.tgz",
  "integrity": "sha512-L557/+sc8ZKJVgo1734HF1QNCxrt/fpqdmdNgySJT+kErux/AJNfPq3flsK0fyJduVmniTutYIMyW48cFoPKDA==",
  "requires": {
    "invariant": "^2.2.4",
    "lodash": "^4.5.0"
  }
},

If I use npm install --force, instead, it adds

"node_modules/expo/node_modules/@unimodules/react-native-adapter": {
  "version": "5.7.0",
  "resolved": "https://registry.npmjs.org/@unimodules/react-native-adapter/-/react-native-adapter-5.7.0.tgz",
  "integrity": "sha512-L557/+sc8ZKJVgo1734HF1QNCxrt/fpqdmdNgySJT+kErux/AJNfPq3flsK0fyJduVmniTutYIMyW48cFoPKDA==",
  "dependencies": {
    "invariant": "^2.2.4",
    "lodash": "^4.5.0"
  },
  "peerDependencies": {
    "react-native": "*",
    "react-native-web": "~0.13.7"
  }
},
"node_modules/expo/node_modules/inline-style-prefixer": {
  "version": "5.1.2",
  "resolved": "https://registry.npmjs.org/inline-style-prefixer/-/inline-style-prefixer-5.1.2.tgz",
  "integrity": "sha512-PYUF+94gDfhy+LsQxM0g3d6Hge4l1pAqOSOiZuHWzMvQEGsbRQ/ck2WioLqrY2ZkHyPgVUXxn+hrkF7D6QUGbA==",
  "peer": true,
  "dependencies": {
    "css-in-js-utils": "^2.0.0"
  }
},
"node_modules/expo/node_modules/react-native-web": {
  "version": "0.13.18",
  "resolved": "https://registry.npmjs.org/react-native-web/-/react-native-web-0.13.18.tgz",
  "integrity": "sha512-WR/0ECAmwLQ2+2cL2Ur+0/swXFAtcSM0URoADJmG6D4MnY+wGc91JO8LoOTlgY0USBOY+qG/beRrjFa+RAuOiA==",
  "peer": true,
  "dependencies": {
    "array-find-index": "^1.0.2",
    "create-react-class": "^15.6.2",
    "deep-assign": "^3.0.0",
    "fbjs": "^1.0.0",
    "hyphenate-style-name": "^1.0.3",
    "inline-style-prefixer": "^5.1.0",
    "normalize-css-color": "^1.0.2",
    "prop-types": "^15.6.0",
    "react-timer-mixin": "^0.13.4"
  },
  "peerDependencies": {
    "react": ">=16.5.1",
    "react-dom": ">=16.5.1"
  }
},

...

  "dependencies": {
    "@unimodules/react-native-adapter": {
      "version": "5.7.0",
      "resolved": "https://registry.npmjs.org/@unimodules/react-native-adapter/-/react-native-adapter-5.7.0.tgz",
      "integrity": "sha512-L557/+sc8ZKJVgo1734HF1QNCxrt/fpqdmdNgySJT+kErux/AJNfPq3flsK0fyJduVmniTutYIMyW48cFoPKDA==",
      "requires": {
        "invariant": "^2.2.4",
        "lodash": "^4.5.0"
      }
    },
    "inline-style-prefixer": {
      "version": "5.1.2",
      "resolved": "https://registry.npmjs.org/inline-style-prefixer/-/inline-style-prefixer-5.1.2.tgz",
      "integrity": "sha512-PYUF+94gDfhy+LsQxM0g3d6Hge4l1pAqOSOiZuHWzMvQEGsbRQ/ck2WioLqrY2ZkHyPgVUXxn+hrkF7D6QUGbA==",
      "peer": true,
      "requires": {
        "css-in-js-utils": "^2.0.0"
      }
    },
    "react-native-web": {
      "version": "0.13.18",
      "resolved": "https://registry.npmjs.org/react-native-web/-/react-native-web-0.13.18.tgz",
      "integrity": "sha512-WR/0ECAmwLQ2+2cL2Ur+0/swXFAtcSM0URoADJmG6D4MnY+wGc91JO8LoOTlgY0USBOY+qG/beRrjFa+RAuOiA==",
      "peer": true,
      "requires": {
        "array-find-index": "^1.0.2",
        "create-react-class": "^15.6.2",
        "deep-assign": "^3.0.0",
        "fbjs": "^1.0.0",
        "hyphenate-style-name": "^1.0.3",
        "inline-style-prefixer": "^5.1.0",
        "normalize-css-color": "^1.0.2",
        "prop-types": "^15.6.0",
        "react-timer-mixin": "^0.13.4"
      }
    }
  }
},

As you see, npm install --force still pins many dependency versions which is stricter.

So for npm 7, if npm install fails because of peer dependency issue, npm install --force is the first thing you might try.

npm: When to use `--force` and `--legacy-peer-deps`

Solution 1:

Solution 2:

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