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

Question

I'm trying to understand how recreating the node_modules directory for deployment works.
We're using npm ci instead of npm install to ensure a clean slate during deployment. However, when we run it without any flags, we get the following error:
Fix the upstream dependency conflict, or retry this command with --force, or --legacy-peer-deps to accept an incorrect (and potentially broken) dependency resolution.
The documentation for npm install for --force is as follows (there are no flags on npm ci's page):
The -f or --force argument will force npm to fetch remote resources even if a local copy exists on disk.
Meanwhile, the documentation for --legacy-peer-deps says:
--legacy-peer-deps: ignore all peerDependencies when installing, in the style of npm version 4 through version 6.
It seems that both flags will let npm ci generate the node_modules directory without any issues, but I am still unclear about the differences between the two.
From what I understand, --force sounds like it will be on a last-dependency-downloaded-wins basis and will overwrite any previously downloaded dependencies. Meanwhile, --legacy-peer-deps sounds like it will always skip peer dependencies (whatever those are) during installation even if there are no issues.
What are the differences between the two flags, and when should we use them?

Answer 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.
--force: will force npm to fetch remote resources even if a local copy exists on disk.

Answer 2

In the article 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: react#17.0.1
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/react-native#7.2.0
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 react-native#0.63.2
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/react-native#7.2.0
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.

Answer 3

For those wondering which is safer, the answer is --force
--legacy-peer-deps ignores peer dependencies entirely, which can screw up your dependency resolution.
--force on the other hand simply sets a different peer dependency version for conflicting dependencies
Using force isn't always ideal though because each dependency version takes up extra space. Using force with many dependencies will increase your total space requirement a decent amount.