How to ensure a value is JSON encodable (Node.js)

ensure_json_encodable.js

const assert = require('assert');

const _ = require('lodash');

const ensureJSONEncodable = (obj, path = null, visited = null) => {
  if (obj === null || _.isBoolean(obj) || _.isNumber(obj) || _.isString(obj)) {
    return;
  }

  path = path || [];

  if (!_.isPlainObject(obj) && !_.isArray(obj)) {
    const typeName = typeof obj;
    const pathStr = path.join('.');
    throw new TypeError(
      `Type '${typeName}' is not JSON-encodable (path: '${pathStr}')`
    );
  }

  visited = visited || new Set();

  if (visited.has(obj)) {
    const pathStr = path.join('.');
    throw new TypeError(
      `Circular structure is not JSON-encodable (path: '${pathStr}')`
    );
  }

  visited.add(obj);

  for (const key in obj) {
    ensureJSONEncodable(obj[key], path.concat(key), visited);
  }
};

const test = () => {
  ensureJSONEncodable(null);
  ensureJSONEncodable(1.23456);
  ensureJSONEncodable('hello');
  ensureJSONEncodable({ foo: 'bar' });
  ensureJSONEncodable({ foo: { bar: 'baz' } });
  ensureJSONEncodable({ foo: { bar: 'baz', items: [null, 1234, {}] } });

  // These object keys are stringified by JSON.stringify(), e.g., 12.34 ->
  // '12.34', so I guess we can consider them JSON-encodable.
  ensureJSONEncodable({ 12.34: 56.78 });
  ensureJSONEncodable({ [null]: true });
  ensureJSONEncodable({ [undefined]: true });

  assert.throws(
    () => {
      ensureJSONEncodable(undefined);
    },
    {
      name: 'TypeError',
      message: "Type 'undefined' is not JSON-encodable (path: '')",
    }
  );
  assert.throws(
    () => {
      ensureJSONEncodable(/hello/);
    },
    {
      name: 'TypeError',
      message: "Type 'object' is not JSON-encodable (path: '')",
    }
  );
  assert.throws(
    () => {
      ensureJSONEncodable(console);
    },
    {
      name: 'TypeError',
      message: "Type 'object' is not JSON-encodable (path: '')",
    }
  );
  assert.throws(
    () => {
      ensureJSONEncodable({ foo: [null, { xyz: undefined }] });
    },
    {
      name: 'TypeError',
      message: "Type 'undefined' is not JSON-encodable (path: 'foo.1.xyz')",
    }
  );
  assert.throws(
    () => {
      ensureJSONEncodable(function () {});
    },
    {
      name: 'TypeError',
      message: "Type 'function' is not JSON-encodable (path: '')",
    }
  );
  assert.throws(
    () => {
      ensureJSONEncodable({ foo: [null, { hello: function () {} }] });
    },
    {
      name: 'TypeError',
      message: "Type 'function' is not JSON-encodable (path: 'foo.1.hello')",
    }
  );
  assert.throws(
    () => {
      const obj = { num: 1 };
      obj.items = [{ self: obj }];
      ensureJSONEncodable(obj);
    },
    {
      name: 'TypeError',
      message:
        "Circular structure is not JSON-encodable (path: 'items.0.self')",
    }
  );
};

test();