JS function to describe the types of the given value (e.g. a JSON object)

buildSchema.js

/**
 * Return an object that describes types of the given value.
 *
 * @param {unknown} value
 * @param {object} schema
 */
function buildSchema(value, schema = {}) {
  if (value === null) {
    schema.nulls = (schema.nulls || 0) + 1;
    return schema;
  }

  if (typeof value === 'string') {
    schema.strings = (schema.strings || 0) + 1;
    return schema;
  }

  if (typeof value === 'number') {
    schema.numbers = (schema.numbers || 0) + 1;
    return schema;
  }

  if (Array.isArray(value)) {
    schema.arrayOf = schema.arrayOf || {};
    value.forEach(el => buildSchema(el, schema.arrayOf));
    return schema;
  }

  if (typeof value === 'object') {
    schema.obj = schema.obj || {};
    for (const [k2, v2] of Object.entries(value)) {
      schema.obj[k2] = schema.obj[k2] || {};
      buildSchema(v2, schema.obj[k2]);
    }

    // Check for missing properties that the schema knows about.
    for (const k3 of Object.keys(schema.obj)) {
      if (!Object.hasOwn(value, k3)) {
        schema.obj[k3].undefineds = (schema.obj[k3].undefineds || 0) + 1;
      }
    }
  }

  return schema;
}