To CI build your node.js app, you can manage your code by a repo on VSTS firstly (or remote repo hosted on other sites such as github, bitbucket ect), then create a build definition definition for your node.js webjob. Detail steps as below:
1. Manage code by VSTS repo (git or TFVC)
You can manage your node.js app to a current repo you used or you can create a new repo to manage your node.js app. Create a new git repo (I use git repo as example, similar with TFVC), you can refer create a git repo for team project.
Push your node.js code to VSTS repo. You can use VS (clone the repo -> copy your code in local repo -> commit -> push) or git commands directly.
2. Create CI build
In Builds Tab -> New -> use the Node.JS With Gulp (PREVIEW) template (or you can use empty template and then add tasks manually) -> Get sources step -> specify repo and branch which your code exists -> specify settings for tasks -> add other tasks if you need -> Triggers Tab -> enable Continuous Integration.

More details for node.js app CI build, you can refer define your CI build for node.js app.
3. Add your app in Azure webjobs
In the CI build definition, add below tasks:
Copy files task: to copy the App you want to run on Azure webjobs from $(System.DefaultWorkingDirectory) to $(Build.ArtifactStagingDirectory).
Archive Files task: create a zip file (such as $(Build.ArtifactStagingDirectory)/webjob.zip) from files in $(Build.ArtifactStagingDirectory).
Azure App Service Deploy task: deploy your zip file ($(Build.ArtifactStagingDirectory)/webjob.zip) to Azure.
And more details, you can also refer this thread.