How do I get my Azure DevOps Pipeline build to fail when my linting script returns an error?

I am using the Azure Pipelines GitHub add-on to ensure that pull requests pass my linting. However, I have just made a test pull request which fails my linting, but the Azure Pipeline succeeds.

Here is my azure-pipelines.yml

# Node.js with React
# Build a Node.js project that uses React.
# Add steps that analyze code, save build artifacts, deploy, and more:
# https://learn.microsoft.com/azure/devops/pipelines/languages/javascript

trigger:
- master

pool:
  vmImage: 'Ubuntu-16.04'

steps:
- task: NodeTool@0
  inputs:
    versionSpec: '8.x'
  displayName: 'Install Node.js'

- script: |
    npm install
    npm run lint # Mapped to `eslint src` in package.json
    npm run slint # `stylelint src` in package.json
    npm run build
  displayName: 'npm install and build'

And here is (part of) the output on a branch which I know fails on npm run lint

> geograph-me@0.1.0 lint /home/vsts/work/1/s
> eslint src


/home/vsts/work/1/s/src/js/components/CountryInput.js
  26:45  error  'onSubmit' is missing in props validation  react/prop-types
  27:71  error  'onSubmit' is missing in props validation  react/prop-types

✖ 2 problems (2 errors, 0 warnings)

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! geograph-me@0.1.0 lint: `eslint src`
npm ERR! Exit status 1 # Exit status 1, yet the build succeeds?
npm ERR! 
npm ERR! Failed at the geograph-me@0.1.0 lint script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/vsts/.npm/_logs/2019-03-16T05_30_52_226Z-debug.log

> geograph-me@0.1.0 slint /home/vsts/work/1/s
> stylelint src


> geograph-me@0.1.0 build /home/vsts/work/1/s
> react-scripts build

Creating an optimized production build...
Compiled successfully.

# Truncated...

As you can see, linter runs nicely and catches my intentional error (I removed a prop type validation), and exits with code 1.

However the build just carries on its merry way.

What do I need to do to make such a linting error stop my build in its tracks and not return success?

Thank you in advance.

this means your script "swallows" the exit code and exits normally. you need to add a check to your script that would catch the exit code of your npm run lint and exit with the same exit code, something like:

- script: |
    npm install
    npm run lint # Mapped to `eslint src` in package.json
    if [ $? -ne 0 ]; then
        exit 1
    fi
    npm run slint # `stylelint src` in package.json
    npm run build