Build and publish pre-releases in Travis CI

Author: Vinnl

.npmignore

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+# This file primarily exists to prevent ./dist from being ignored when publishing.
+# (Because it's listed in .gitignore).
+
+coverage

.travis.yml

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+language: node_js
+install:
+  - npm install
+script:
+  - npm test
+  # Make sure it still builds successfully - the build isn't actually used yet:
+  - npm run build
+before_deploy:
+  - node ./travis/bumpversion.js
+deploy:
+  provider: npm
+  # Do not throw away the updated package.json we generated in `before_deploy`:
+  skip_cleanup: true
+  email: "$NPM_EMAIL"
+  api_key: "$NPM_TOKEN"
+  # Note: do not deploy on pull request, because $TRAVIS_BRANCH will be the target branch.
+  tag: "$TRAVIS_BRANCH"
+  on:
+    all_branches: true

build.sh

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+/**
+ * npm does not allow you to publish a package with the same version multiple times.
+ * Thus, to publish prerelease versions tagged to the branch they're built from,
+ * we need to generate unique version numbers that are also higher than versions already published.
+ * To achieve this, we append `build<build_number>` to the version number.
+ * The actual release version can eventually be published without the suffix.
+ */
+
+if (!process.env.TRAVIS_BUILD_NUMBER || process.env.TRAVIS_BUILD_NUMBER.length === 0) {
+  console.error('Could not read the build number to bump the package version - aborting publish.')
+  process.exit(1)
+}
+
+const fs = require('fs')
+const path = require('path')
+
+const packageJson = require('../package.json')
+packageJson.version = `${packageJson.version}build${process.env.TRAVIS_BUILD_NUMBER}`
+fs.writeFileSync(path.resolve(__dirname, '../package.json'), JSON.stringify(packageJson))