JavaScript Error handling and control-flow branching


Example

Callbacks are often used to provide error handling. This is a form of control flow branching, where some instructions are executed only when an error occurs:

const expected = true;

function compare(actual, success, failure) {
  if (actual === expected) {
    success();
  } else {
    failure();
  }
}

function onSuccess() {
  console.log('Value was expected');
}

function onFailure() {
  console.log('Value was unexpected/exceptional');
}

compare(true, onSuccess, onFailure);
compare(false, onSuccess, onFailure);

// Outputs:
//   "Value was expected"
//   "Value was unexpected/exceptional"

Code execution in compare() above has two possible branches: success when the expected and actual values are the same, and error when they are different. This is especially useful when control flow should branch after some asynchronous instruction:

function compareAsync(actual, success, failure) {
  setTimeout(function () {
    compare(actual, success, failure)
  }, 1000);
}

compareAsync(true, onSuccess, onFailure);
compareAsync(false, onSuccess, onFailure);
console.log('Doing something else');

// Outputs:
//   "Doing something else"
//   "Value was expected"
//   "Value was unexpected/exceptional"

It should be noted, multiple callbacks do not have to be mutually exclusive – both methods could be called. Similarly, the compare() could be written with callbacks that are optional (by using a noop as the default value - see Null Object pattern).