Compare the result of a new function against an existing function.

const findCatsById = twinning({
  name: "findCatsById",
  oldFn: OldDatabase.findCatsById,
  newFn: NewDatabase.findCatsById,
  onDiff: onDiffs,
  onError: onError
});

findCatsById("1234", function(err, result) {
  /*
    `err` and `result` come from `OldDatabase.findCatsById`,
    If either function errors, their errors will be passed to `onError` (below).
    If no error occurs, but results are different, diffs will be passed to `onDiffs` (below).
    If either `onDiffs` or `onError` throws, that will become the error.
  */
});

function onDiffs (name, diffs) {
  diffs.forEach(function(diff) {
    console.log("Different result for", name, JSON.stringify(diff));
  });
}

function onError (name, oldError, newError) {
  if (oldError) {
    console.error("Error for old function in", name, oldError);
  }
  if (newError) {
    console.error("Error for new function in", name, oldError);
  }
}

$ npm install twinning

• name (required): A label for this comparison. This so you can re-use onDiffs or onError between multiple comparison.
• oldFn (required): The original function. It's assumed that this function is currently being used in production, and the results can be trusted. The function must take a callback as its last argument unless promises or sync is specified. Both this function and newFn must complete before the comparison will complete.
• newFn (required): The new function to compare. We assume this function is not yet reliable, so its results will be thrown away after the comparison. The function should match the type (callback, promise, synchronous) of oldFn. Both this function and oldFn must complete before the comparison will complete.
• onDiffs: A function that will be called when newFn yields a different result than oldFn, but neither function errors. You might want to use this function to log differences, or perhaps throw. If this function throws, the comparison will return/yield that error. onDiffs will be called with the following arguments:
  • name: See above.
  • diffs: An array of change records between the results of oldFn and newFn. We've used deep-diff to implement the comparison; see their API for an overview of the structure of change records.
• onError: A function that will be called when either function errors. If onError is called, onDiffs will not be. You might want to use this option to log or throw the error. If this function throws, the comparison will return/yield that error. onError will be called with the following arguments
  • name: See above.
  • oldError: The error, if any, from oldFn.
  • newError: The error, if any, from newFn.
• before: If provided, this function will be called with the provided arguments. Its return value will then be used as the argument to oldFn and newFn. Note that this means that if a before block is used, oldFn and newFn can only take a single argument. If before throws an error, oldFn and newFn will not be run.
• after: If provided, this function will be called with the result of the comparison, and its return value will be returned instead. If an error occurs, after will not be called.
• disabled: If true, newFn will not be called, and no diffs will be calculated. oldFn, and before and after if provided, will still be called.
• promises: Set this to true if your oldFn and newFn return a promise instead of using callbacks.
• sync: Set this to true if youre oldFn and newFn are synchronous and do not use callbacks.

The defaults method returns a function that is pre-configured with certain options. This function can then be used exactly like the base method.

const twinning = require("twinning");
const withDefaults = twinning.defaults({
  onDiffs: myDiffLogger,
  onError: myErrorHandler
});
const findCatsById = withDefaults({
  name: "findCatsById",
  oldFn: OldDatabase.findCatsById,
  newFn: NewDatabase.findCatsById
});