Would be possible to watch only one function?

[mastazi]

Currently I have npm run watch set to run webpack-cli -w and my webpack.config.js is the same as the one shown at the beginning of the README here on Github.

Currently in our SAM project my dev machine takes more than a minute to build. So I was wondering if there was a way to create a command where I can run npm run watch myFunction and it will only watch that single function? Usually when doing dev work I'm often only working on one single function at a time so this would be super helpful.

I suspect that there is a way but my knowledge of Webpack & of the aws-sam-webpack-plugin is not great. Any ideas?

[mastazi]

Replying to myself...

I am now able to do the following:

Assuming that:

1. the handler is in src/handlers/users/create.ts and
2. the function name (in template.yml) is createUser,

then I can build like this:
npm run build -- --env handler=users/create.ts --env functionName=createUser
and I can watch with:
npm run watch -- --env handler=users/create.ts --env functionName=createUser

This assumes that all handlers are in src/handlers. If you don't have a handlers folder then modify the line starting with entry: below.

const path = require("path");
const AwsSamPlugin = require("aws-sam-webpack-plugin");

module.exports = (env) => {
  let awsSamPlugin = {}
  const singleFunction = (env.handler && env.functionName);
  if (singleFunction) {
    awsSamPlugin = {
      entry: () => '',
      filename: () => ''
    }
  } else {
    awsSamPlugin = new AwsSamPlugin();
  }
  return {
    // Loads the entry object from the AWS::Serverless::Function resources in your
    // SAM config. Setting this to a function will
    entry: singleFunction ? './src/handlers/' + env.handler : () => awsSamPlugin.entry(),

    // Write the output to the .aws-sam/build folder
    output: {
      filename: singleFunction ? './.aws-sam/build/' + env.functionName + '/app.js' : (chunkData) => awsSamPlugin.filename(chunkData),
      libraryTarget: "commonjs2",
      path: path.resolve(".")
    },

    // Create source maps
    devtool: "source-map",

    // Resolve .ts and .js extensions
    resolve: {
      extensions: [".ts", ".js"]
    },

    // Target node
    target: "node",

    // AWS recommends always including the aws-sdk in your Lambda package but excluding can significantly reduce
    // the size of your deployment package. If you want to always include it then comment out this line. It has
    // been included conditionally because the node10.x docker image used by SAM local doesn't include it.
    externals: process.env.NODE_ENV === "development" ? [] : ["aws-sdk"],

    // Set the webpack mode
    mode: process.env.NODE_ENV || "production",

    // Add the TypeScript loader
    module: {
      rules: [{ test: /\.tsx?$/, loader: "ts-loader" }]
    },

    // Add the AWS SAM Webpack plugin
    plugins: singleFunction ? [] : [awsSamPlugin]
  }
};

[mastazi]

@graphboss I saw the source of aws-sam-webpack-plugin and I think this could be achieved by adding an optional parameter to entry like so:

awsSamPlugin.entry(functionName)

then it would be possible to do

npm run watch -- --env functionName=myFunction

thus eliminating the need to create two env variables (since aws-sam-webpack-plugin is already able to find the path of each handler).

If you think this is a viable change I can make a PR.