JSON to JSON Schema Script

json-to-json-schema.js

#!/usr/bin/env node

const fs = require('fs')
// NOTE: this package didn't work for me
// const toJsonSchema = require('to-json-schema')
var stringify = require('json-stable-stringify')
const Ajv = require('ajv');

const ajv = new Ajv();

function isObject (value) {
  return value != null && typeof value === 'object' && value.constructor === Object
}

function toSchema(data) {
  if (isObject(data)) {
    return {
      type: 'object',
      properties: Object.keys(data).reduce((properties, key) => {
        properties[key] = toSchema(data[key])
        return properties
      }, {})
    }
  } else if (Array.isArray(data)) {
    return {
      type: 'array',
      items: toSchema(data[0])
    }
  } else {
    return { type: (typeof data) }
  }
}

async function main() {
  const data = JSON.parse(fs.readFileSync(0))
  const schema = toSchema(data)

  const isValidSchema = ajv.validateSchema(schema)
  if (!isValidSchema) {
    console.log('WARNING! schema is not valid!')
  }
  const validate = ajv.compile(schema)
  const valid = validate(data)
  if (!valid) {
    console.log('WARNING! schema errors:', stringify(validate.errors, { space: 2}))
  }

  console.log(stringify(schema, { space: 2 }))
}

main()