How can I write multiline scripts in npm scripts?

You can chain independent tasks.

Here is an example:

"scripts": {
    "lint-jshint": "jshint --verbose --show-non-errors ./src/main/js",
    "lint-eslint": "eslint ./src/main/js ./src/test/js",
    "lint-csslint": "csslint ./src/main/js",

    "lint": "npm run -s lint-jshint & npm run -s lint-eslint & npm run -s lint-csslint",

    "pretest": "rimraf ./build/reports/tests && mkdirp ./build/reports/tests && npm run -s lint",
    "test": "karma start ./src/test/resources/conf/karma.conf.js",
    ...

Here is a nice blog which I used at that time: https://www.keithcirkel.co.uk/how-to-use-npm-as-a-build-tool/

You can't do that.

The following code is in read-json.js which is in package node_modules/npm/node_modules/read-package-json which is used in run-script.js to execute $ npm run-script ~~ or $ npm run ~~ which is its alias.

function scriptpath (file, data, cb) {
  if (!data.scripts) return cb(null, data)
  var k = Object.keys(data.scripts)
  k.forEach(scriptpath_, data.scripts)
  cb(null, data)
}

function scriptpath_ (key) {
  var s = this[key]
  // This is never allowed, and only causes problems
  if (typeof s !== 'string') return delete this[key]

  var spre = /^(\.[\/\\])?node_modules[\/\\].bin[\\\/]/
  if (s.match(spre)) {
    this[key] = this[key].replace(spre, '')
  }
}

The key in scriptpath_ is like "build:server" in your code.

The this[key] is like "./node_modules/babel-cli/bin/babel.js . -d dist/server --ignore node_modules,dist,client,public,webpack*" in your code.

So, if you write the code which is not string type, in other words, if you don't write the string text in package.json, it will be an error unless you contribute to the package npm/read-package-json.

Another common alternative is to write an npm command that references a local bash script (where you have more power to do what you want).

i.e.

# package.json
{
  "name": "project",
  "version": "1.0.0",
  "description": "",
  "main": "server.js",
  "scripts": {
    "lint": "./node_modules/eslint/bin/eslint.js --format \"./node_modules/eslint-friendly-formatter/index.js\" .",
    "build:server": "./build-server.sh"
  }
}

# build-server.sh
#!/bin/bash

./node_modules/babel-cli/bin/babel.js . \
  -d dist/server \
  --ignore \
    node_modules,\
    dist,\
    client,\
    public,\
    webpack*

NOTE: make sure you give yourself permission to run the file; otherwise you'll run into permission issues

sudo chmod 755 'build-server.sh'

See: Run script on mac prompt "Permission denied"