index.js

#!/usr/bin/env node
const args = process.argv.slice(2)

if (args.length < 3)
  console.error(`usage: node ${process.argv[1]} <file> <key> [<subkey> ...] <value>`)

const file = args.shift()
const value = args.pop()

const { readFileSync, writeFileSync } = require('fs')
const jsonParse = require('json-parse-even-better-errors')
const kIndent = Symbol.for('indent')
const kNewline = Symbol.for('newline')

const getData = file => {
  try {
    const data = jsonParse(readFileSync(file, 'utf8'))
    if (!data || typeof data !== 'object' || Array.isArray(data))
      throw new Error('not an object')
    return data
  } catch (er) {
    console.error('file borked or missing, clobbering it', er.message)
    return { [kIndent]: 2, [kNewline]: '\n' }
  }
}

const data = getData(file)
const { [kIndent]: indent, [kNewline]: newline } = data

let o = data
const finalKey = args.pop()
for (const key of args) {
  console.error(o, key)
  if (typeof o[key] === 'object' && o[key])
    o = o[key]
  else
    o = o[key] = Object.create(null)
}
o[finalKey] = value

writeFileSync(file, JSON.stringify(data, null, indent).split('\n').join(newline) + newline)