03-callbacks-function-errors-handling.md

title	permalink	excerpt	last_modified_at
Callbacks function errors handling	/the-modern-javascript-tutorial/callbacks-function-errors-handling		2018-06-12 15:58:49 -0400

In the previous chapter we didn't consider errors. What if the script loading fails?

Here's an improved version of loadExternalScript that tracks loading errors:

function loadExternalScript(src, callback) {
  let script = document.createElement('script');
  script.src = src;

  script.onload = () => callback(null, script);
  script.onerror = () => callback(new Error(`Script load error for ${src}`));

  document.head.append(script);
}

It calls callback(null, script) for successful load and callback(error) otherwise.

Here's the usage:

loadExternalScript('script.js', function(error, script) {
  if (error) {
    // handle error
  } else {
    // script loaded successfully
  }
});

It's called the "error-first callback" style.

The convention is:

1. The first argument of callback is reserved for an error if it occurs. Then callback(err) is called.
2. The second argument (and the next ones if needed) are for the successful result. Then callback(null, result1, result2...) is called.

So the callback function is used both for reporting errors and passing back results.