lightDeprecate.js

exports.deprecate = function deprecate(fn, msg, code) {

  // pre wrapper checks go here
    
  function deprecated() {
    if (!warned) {
      warned = true;
      if (code !== undefined) {
        process.emitWarning(msg, 'DeprecationWarning', code, deprecated);
      } else {
        process.emitWarning(msg, 'DeprecationWarning', deprecated);
      }
    }
    if (new.target) {
      return Reflect.construct(fn, arguments, new.target);
    }
    return fn.apply(this, arguments);
  }

  // The wrapper will keep the same prototype as fn to maintain prototype chain
  Object.setPrototypeOf(deprecated, fn);
  if (fn.prototype) {
    // Setting this (rather than using Object.setPrototype, as above) ensures
    // that calling the unwrapped constructor gives an instanceof the wrapped
    // constructor.
    deprecated.prototype = fn.prototype;
  }

  return deprecated;
};