Unable to resolve dependency tree error in Angular while installing npm packages

To fix Unable to resolve dependency tree error in Angular while installing npm packages follow the below steps.

1. Run npm install --save --legacy-peer-deps instead of npm install command.
2. Another way is delete node_modules folder and package_lock.json file and then run npm cache clean --force after npm i --force command.

It’s a dependency conflict (incorrect and potentially broken dependency) error, so run the command with --force or --legacy-peer-deps flag.

npm install --save --legacy-peer-deps

or 

npm cache clean --force
npm i --force

Another temporary solution is using prior versions of the Node.js or downgrading npm.

Set the --legacy-peer-deps flag to always true

And instead of passing --legacy-peer-deps flag each time we can set the npm config to true.

npm config set legacy-peer-deps true

Then clean the cache and run npm install

npm cache clean --force

npm install

This the best option if npm install is run indirectly, i.e, by a script.

Downgrading npm version

When using npm 7, Unable to resolve dependency tree errors comes up a lot because peer dependencies issues are treated as errors in version 7.

But these errors are only warnings in version 6.

So to resolve this error set --legacy-peer-deps flag to true in npm 7 version.

If the above config is not working, downgrade npm 7 to npm 6.

Downgrading Node.js is not necessary.

Because the relevant dependency management code is in npm.

Downgrading Node.js will often work automatically, because doing so will downgrade npm as well.

Instead of downgrading npm, use npx to use the previous version of npm for just the install command as shown below.

npx -p npm@6 npm install

What is Unable to resolve dependency tree error?

Here is the sample unable to resolve dependency tree error in Angular.

npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR!
npm ERR! While resolving: project-admin@11.0.0
npm ERR! Found: @angular/common@11.0.3
npm ERR! node_modules/@angular/common
npm ERR!   @angular/common@"11.0.3" from the root project
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! peer @angular/common@"^9.1.0 || ^10.0.0" from @agm/core@3.0.0-beta.0
npm ERR! node_modules/@agm/core
npm ERR!   @agm/core@"3.0.0-beta.0" from the root project

First we will understand the problem.

In the above error @agm/core@3.0.0-beta.0 requires @angular/common version ^9.1.0 or ^10.0.0.

And the actual @angular/common version 11.0.3.

The problem is about peer dependencies.

So The solution is to downgrade @angular/common. And that’s not advisable.

So use legacy dependencies logic for installing packages using --legacy-peer-deps.

And to install legacy peer dependencies automatically set the npm config --legacy-peer-deps to true.

npm config set legacy-peer-deps true