How to increase a version of an npm package using Azure Devops pipeline

Don't use the files in the source repo to keep track of the current/next version. I don't think you can break the loop easily, if you do.

You might be able to get away with running npm --no-git-tag-version version to increment the package.json version inside the build agent without the commit, so that you don't have a change you would have to push back to the origin. It should just change package.json and leave it dirty.

Wait until after the build has succeeded. Use a custom script task to extract the version from package.json, then git reset --hard (there is no reason to keep anything that has changed on the build server). While this will undo the change in package.json, you can now create a tag on the head that contains that version, then do a git push origin {tag-name} which should not introduce a new commit on the origin which would then re-trigger your pipeline.

Actually, I don't think a pipeline will be triggered just because a tag was added, but I honestly haven't tested it. I'm pretty sure it won't.

Sequence of tasks in your pipeline:

[Given a source repo where you have made sure to set the major and minor versions in package.json to reflect the current state of the code according to the rules of Semantic Versioning, and leave the patch value always set to 0, and, if desired, using pre-* values to describe the quality of the major/minor value:]

1. (This is not really a task, but just describing the start of the execution:) The job starts. Automatically, the code is pulled from the source repo into the build agent.
2. Using a utility command-line task and code or script that you write, run git describe --tags to find the most recent tag with a tagname matching the pattern you use (see below). (I prefer this sequence over calling npm version from-git because npm will just use the latest tag, which might not be a version number, depending on how much control you have over the branch.) Use string or regular expression operations to extract the major, minor, patch, and whatever pre-* value you might have. This is the previous version that we'll use to compare with what's in the package.json file. Note that you may have to follow these instructions to run a git command. Save it to a pipeline variable.

3. Using a utility command-line task and code or script that you write, run npm version to get the current major/minor/pre version out of the package.json file and save it to a different pipeline variable. I'm using PowerShell Core, so my command would look something like this to create a "currentPackageVersion" pipeline variable:

   & npm.cmd version | ConvertFrom-Json | Select-Object -ExpandProperty {name-of-your-package} | Set-Variable -Name 'packageVersion' | Write-Output "#vso[task.setvariable variable=currentPackageVersion]$packageVersion"
   

4. Using a utility command-line task and code or script that you write, compare the previous version's major, minor, and pre-* values to determine whether any of them have changed. Set a new pipeline variable to reflect whether it has changed or not. I'll use the name "restartVersionPatchNumber", which is true if the current major, minor, or pre-* values are different from the previous version's major, minor, or pre-* values.

5. The npm Task conditionally runs a custom command: npm --no-git-tag-version version patch, which updates package.json in the build agent but does not commit the change, leaving your Working Area modified (dirty) (which might cause issues on subsequent builds if you are using your own build agents instead of hosted agents). The condition expression of the Task uses a custom condition that evaluates to the variable that I just set in the previous step ("restartVersionPatchNumber"). Note that if this task does not run, it should just use the value of the version that is in the package.json file (the current version that is now in the "currentPackageVersion" pipeline variable).
6. Using a utility command-line task and code or script that you write, run npm version to extract the new version that the npm version command set. Save it to a new pipeline variable; I'll call it "newVersionNumber".
7. The regular build tasks run, producing artifacts and possibly publishing them
8. Using a utility command-line task, run git reset --hard. You'll need to do this even if you're using a hosted build agent, because of the next step.
9. Using a utility command-line task, create a variable from the saved version number ("newVersionNumber") that contains the value of the tag's tagname that you want to use. Use a distinctive patter, like "AzPipelineBuild-PipelineName-PackageVersion-1.0.0" but using your version instead of 1.0.0.
10. Using a utility command-line task, run git tag {*tagname*}. For PowerShell, the syntax would be & git.exe tag $env:newVersionNumber
11. Using a utility command-line task, run git push origin {*tagname*}
12. Profit

You could (and maybe should) combine the command-line steps. I'm really partial to the PowerShell task running PowerShellCore (pwsh) as you may have guessed.

The package that got created as an artifact of the job will have the updated version as it was created in the build, and it will match the tag that is now in your original source code repo.

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Alternately, use an external source (an Azure Function, etc.) or a different (second) git repo or even another branch that isn't tied to your CI trigger in your project that you use just for tracking build numbers, then use token replacement tasks to set the version just before you start the build. I don't like this idea much, but it would prevent re-triggering a new build.

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Also, you're hopefully building just one package with that repo. The next thing to do is to publish that package to your Azure Artifacts feed, or npmjs.org or wherever. Don't rely on having the version baked into the original source code; anything that depends on that package should be pulling it out of that feed you published it to, not relying on it being built earlier in the build steps with a new version number.

Here's my implementation in Powershell which is a complete step based on the accepted answer. Note that this also handles some of the git issues such as the detached head that comes with this type of approach.

steps:
- checkout: self
  persistCredentials: true
.
.
.
    - task: PowerShell@2
      displayName: 'Bump the version'
      inputs:
        targetType: 'inline'
        script: |
          $BranchName = "$(Build.SourceBranch)" -replace "refs/heads/"
          git checkout $BranchName
          git config --global user.email "[email protected]"
          git config --global user.name "Your name"
          npm version prerelease -m "Auto increment pre-release version to %s [skip ci]" --force
          git push

I've added the same tasks as mentioned in the question with a small change.
Apparently there is a way to skip the pipeline triggering, see here

So the npm version task looks like this:

version patch -m "Bump version to %s [skip ci]" --force

which prevents the next build from being triggered.

TIP: remember to give the 'author' (Azure DevOps user) permissions to Bypass policies when pushing if any.