> ## Documentation Index > Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt > Use this file to discover all available pages before exploring further. # Demo Buildpack Buildplan > This article demonstrates updating detect and build scripts to dynamically set the Node.js version from the package.json file. In this article, we demonstrate how to update your detect and build scripts to dynamically set the Node.js version specified by the user in the package.json file. The package.json file includes an "engines" section where you can declare the required Node.js version. In our example, the package.json file specifies Node.js version 18.18.0. *** ## Updating the Detect Script The detect script now sets a default Node.js version and then attempts to extract the user-specified version from the package.json file by reading the "engines.node" property. Finally, it writes the appropriate "provides" and "requires" fields to the buildpack build plan, which the build script uses later to download and install the correct Node.js version. ```bash theme={null} if [[ -f package.json ]]; then # default version version=18.18.1 # Determine Node.js version from package.json using jq version=$(jq -r '.engines.node' "./package.json") # echo "version: ${version}" cat > "${CNB_BUILD_PLAN_PATH}" << EOL provides = [{ name = "node-js" }] requires = [{ name = "node-js", metadata = { version = "${version}" } }] EOL fi ``` Ensure that your package.json file contains the "engines" section with the correct Node.js version. To test the container state, run: ```bash theme={null} docker ps ``` Expected output for `docker ps`: ```bash theme={null} CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 28f480564d5e myapp "/cnb/process/web" 2 seconds ago Up 2 seconds 0.0.0.0:8000->8080/tcp, [::]:8000->8080/tcp eager ``` Verify the application response with: ```bash theme={null} curl localhost:8000 ``` Output: ```bash theme={null} Hello, World! ``` *** ## Updating the Build Script Previously, the build script hardcoded Node.js version 18.18.1. In the updated script, a default Node.js version (18.18.0) is defined, and the desired version is later retrieved from the build plan. This retrieved version is then used to dynamically construct the download URL for the appropriate Node.js tarball. Below is the revised build script: ```bash theme={null} #!/usr/bin/env bash set -euo pipefail echo "Building image using my-js-buildpack buildpack" default_node_js_version="18.18.0" echo "--> Downloading and extracting NodeJS" node_js_url=https://nodejs.org/dist/v18.18.1/node-v18.18.1-linux-x64.tar.xz wget -q -O - "${node_js_url}" | tar -xJf - --strip-components 1 export PATH="./bin:$PATH" pwd ls -la echo "--> Installing Application Dependencies" ``` After building, you can check the container status with: ```bash theme={null} docker ps ``` Expected output: ```bash theme={null} CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 28f480564d5e myapp "/cnb/process/web" 2 seconds ago Up 2 seconds 0.0.0.0:8000->8080/tcp, [::]:8000->8080/tcp eager ``` And confirm the application response again: ```bash theme={null} curl localhost:8000 ``` Output: ```bash theme={null} Hello, World! ``` *** ## Dynamically Constructing the Node.js Download URL The build script is now updated to dynamically include the user-specified Node.js version. After retrieving the version from the build plan, the script prints the version for verification and constructs the download URL based on that version. The snippet below shows the modified section: ```bash theme={null} echo "nodejs version: ${node_js_version}" echo "--> Downloading and extracting NodeJS" node_js_url=https://nodejs.org/dist/v${node_js_version}/node-v${node_js_version}-linux-x64.tar.xz wget -q -O - "${node_js_url}" | tar -xJf - --strip-components 1 export PATH="./bin:$PATH" pwd ls -la echo "--> Installing Application Dependencies" npm ci echo "CNB_LAYERS_DIR: ${CNB_LAYERS_DIR}" cat "${CNB_LAYERS_DIR}/launch.toml" << EOL [[processes]] type = "web" EOL ``` Run the container check again with: ```bash theme={null} docker ps ``` Expected output: ```bash theme={null} CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 28f480564d5e myapp "/cnb/process/web" 2 seconds ago Up 2 seconds 0.0.0.0:8000->8080/tcp eager ``` Verify the endpoint: ```bash theme={null} curl localhost:8000 ``` Output: ```bash theme={null} Hello, World! ``` Remember to delete the old container and rebuild the image to ensure the changes take effect. *** ## Verifying the Buildpack with package.json Once again, note that the version requested (as printed in the logs) is 18.18.0. The updated package.json looks like this: ```json theme={null} { "name": "example-application", "version": "1.0.0", "main": "index.js", "scripts": { "test": "echo \"Error: no test specified\" && exit 1" }, "author": "", "license": "ISC", "description": "", "engines": { "node": "18.18.0" }, "dependencies": { "express": "^4.21.1", "uuid": "^11.0.2" } } ``` The console output during the build might resemble: ```plaintext theme={null} [analyzer] Restoring data for SBOM from previous image ==> DETECTING [detector] my-js-buildpack 0.0.1 ==> RESTORING [builder] Building image using my-js-buildpack buildpack [builder] nodejs version: 18.18.0 ==> Downloading and extracting NodeJS [builder] workspace Total 812 [builder] drwxr-xr-x 1 root root 4096 Nov 4 04:42 .. ``` After confirming this output, delete the old container and rebuild the image. The build output will then verify that the correct Node.js version is used. To test the dynamic version retrieval, update the package.json "engines.node" value to 18.18.1 and rebuild the image. The build logs should now indicate that Node.js version 18.18.1 is installed: ```json theme={null} { "name": "example-application", "version": "1.0.0", "main": "index.js", "scripts": { "test": "echo \"Error: no test specified\" && exit 1" }, "author": "", "license": "ISC", "description": "", "engines": { "node": "18.18.1" }, "dependencies": { "express": "^4.21.1", "uuid": "^11.0.2" } } ``` Console output for the new build may include: ```plaintext theme={null} [exporter] Reusing layer 'buildpacksio/lifecycle:process-types' [exporter] Adding label 'io.buildpacks.lifecycle.metadata' [exporter] Adding label 'io.buildpacks.build.metadata' [exporter] Setting default process type 'web' [exporter] Saving myapp... [exporter] *** Images (5d6facb4612d): myapp Successfully built image myapp ``` And later: ```plaintext theme={null} [exporter] Reusing layer 'buildpacksio/lifecycle:process-types' [exporter] Adding label 'io.buildpacks.lifecycle.metadata' [exporter] Adding label 'io.buildpacks.build.metadata' [exporter] Adding label 'io.buildpacks.project.metadata' [exporter] Setting default process type 'web' [exporter] Saving myapp... [exporter] *** Images (56dfacb4612d): [exporter] myapp Successfully built image myapp root in ubuntu-s-4vcpu-8gb-nyc1-01 in ~/buildpack-demo took 20s ``` This confirms that the buildpack now successfully supports dynamic Node.js version selection based on the user's specification in package.json. Happy building! *** ## Additional Resources | Resource Type | Use Case | Example | | ------------- | -------------------- | ------------------------------------------------------ | | Buildpack | Custom build process | [Buildpack Documentation](https://buildpacks.io/docs/) | | Docker | Container management | [Docker Documentation](https://docs.docker.com/) | | Node.js | JavaScript runtime | [Node.js Downloads](https://nodejs.org/en/download/) | For further details, visit the following helpful resources: * [Kubernetes Basics](https://kubernetes.io/docs/concepts/overview/what-is-kubernetes/) * [Docker Hub](https://hub.docker.com/) * [Terraform Registry](https://registry.terraform.io/)