How do you install angular language service in the project?

You can install Angular Language Service in your project with the following npm command,

npm install --save-dev @angular/language-service

After that add the following to the "compilerOptions" section of your project's tsconfig.json

"plugins": [
    {"name": "@angular/language-service"}
]

Note: The completion and diagnostic services works for .ts files only. You need to use custom plugins for supporting HTML files.