JS serializer

serializer.js

function serialize(source) {
  const refs = [];
  const assignments = [];

  function checkRef(current) {
    const index = refs.indexOf(current);
    if (index === -1) {
      refs.push(current)
      return { first: true, index: refs.length - 1};
    }
    return { first: false, index };
  }

  function getRef(index) {
    return `r[${index}]`;
  }

  function createAssignment(line) {
    assignments.push(line + ',');
  }

  function traverse(current) {
    if (current === true) {
      return '!0';
    }
    if (current === false) {
      return '!1';
    }
    if (typeof current === 'number' || current instanceof RegExp) {
      return `${current}`;
    }
    if (current === undefined) {
      return 'void 0';
    }
    if (current === null) {
      return 'null';
    }
    if (typeof current === 'string') {
      return JSON.stringify(current);
    }
    const { first, index } = checkRef(current);
    const ref = getRef(index);
    if (!first) {
      return ref;
    }
    if (current instanceof Date) {
      createAssignment(`${ref}=new Date("${current.toISOString()}")`);
      return ref;
    }
    if (current instanceof Set) {
      createAssignment(`${ref}=new Set()`);
      for (const value of current.keys()) {
        createAssignment(`${ref}.add(${traverse(value)})`);
      }
      return ref;
    }
    if (current instanceof Map) {
      createAssignment(`${ref}=new Map()`);
      for (const [key, value] of current.entries()) {
        createAssignment(`${ref}.set(${traverse(key)},${traverse(value)})`);
      }
      return ref;
    }
    if (Array.isArray(current)) {
      createAssignment(`${ref}=new Array(${current.length})`);
      for (let i = 0, len = current.length; i < len; i += 1) {
        if (i in current) {
          createAssignment(`${ref}[${i}]=${traverse(current[i])}`);
        }
      }
      return ref;
    }
    if (current && typeof current === 'object') {
      createAssignment(`${ref}={}`);
  
      for (const [key, value] of Object.entries(current)) {
        const check = Number(key);
        // Test if key is a valid number or JS identifier
        // so that we don't have to serialize the key and wrap with brackets
        if (check === check || /^([$A-Z_][0-9A-Z_$]*)$/i.test(key)) {
          createAssignment(`${ref}.${key}=${traverse(value)}`);
        } else {
          createAssignment(`${ref}[${JSON.stringify(key)}]=${traverse(value)}`);
        }
      }
      return ref;
    }
  }

  const result = traverse(source);
  const header = assignments.join('');

  return `(r=>(${header}${result}))([])`;
}