Starting a promise chain to handle exceptions correctly

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Here’s a little promises quirk that I’ve overlooked for a while. Generally we all write promises like this:

doSomething()
  .then(doSomethingElse)
  .then(doSomethingFurther)
  .catch(handleError);

And we know that any exception raised will skip to the catch handler. Except for the initial doSomething() call. If an exception is raised here it will be treated like a normal exception and stop executing all the rest of your code. So how do you deal with it? You could:

1. Ensure that doSomething always returns a promise, e.g. wrap the whole thing in a new Promise(function (resolve, reject) { .. }). While this isn’t a bad practice it requires the calling code to assume the function has been implemented correctly, not something I’d rely on.
2. Wrap the start of the promise chain in a promise like this:

   new Promise(function (resolve, reject) {
    resolve(doSomething());
     })
     .then(doSomethingElse)
     .then(doSomethingFurther)
     .catch(handleError);
   

3. Start the promise chain with a dummy promise like this (my preferred approach):

   Promise.resolve()
     .then(doSomething)
     .then(doSomethingElse)
     .then(doSomethingFurther)
     .catch(handleError);
   

   or in angular:

   $q.when()
     .then(doSomething)
     .then(doSomethingElse)
     .then(doSomethingFurther)
     .catch(handleError);
   

• #javascript
• #promises