Question: What is the difference between these four promises?

doSomething().then(function () { return doSomethingElse(); });

doSomething().then(function () { doSomethingElse(); });

doSomething().then(doSomethingElse());

doSomething().then(doSomethingElse);

Question: What is the difference between these four promises?

doSomething().then(function () { return doSomethingElse(); });

doSomething().then(function () { doSomethingElse(); });

doSomething().then(doSomethingElse());

doSomething().then(doSomethingElse);

• javascript
• ecmascript-6
• es6-promise

• 2 Depends what doSomethingElse() returns. What do you think the differences are? – charlietfl Commented Mar 31, 2018 at 20:46
• You should also consider doSomething().then(function(value) { return doSomethingElse(value); }) (hint, hint: that's equivalent to the last case) – Bergi Commented Mar 31, 2018 at 21:05

2 Answers 2

Option 1:

doSomething().then(function () { return doSomethingElse(); });

This executes doSomething(), then when it resolves, it executes doSomethingElse() and the resolved value of the promise chain is the resolved value or return value of doSomethingElse(). No arguments are passed to doSomethingElse().

If you were to summarize this option, you'd say it was:

Chained in Sequence, with no parameter passing.

Option 2:

doSomething().then(function () { doSomethingElse(); });

Same as option 1, except the resolved value of the promise chain is undefined because there is no return value from the .then() handler. Any return value from doSomethingElse(), whether a promise or a value is ignored.

If you were to summarize this option, you'd say it was:

Executed in Sequence (not chained), with no parameter passing.

Option 3:

doSomething().then(doSomethingElse());

This is pretty much never what you want. This executes doSomething() and then, before doSomething() has resolved, it also executes doSomethingElse() and passes the return value from doSomethingElse() as the .then() handler. This is a coding mistake unless doSomethingElse() returns a function to be used as the .then() handler.

If you were to summarize this option, you'd say it was:

Executed Immediately, Probably a Bug

Option 4:

doSomething().then(doSomethingElse);

This is the same as option 1 except that the resolved value from doSomething() is passed as the first argument to doSomethingElse(val). This is structurally the same as this:

doSomething().then(function(result) { return doSomethingElse(result)});

If you were to summarize this option, you'd say it was:

Chained in Sequence, with parameter passing.

Other Notes:

1. If doSomethingElse() returns a promise, then only Options 1 and 4 will add that promise to the chain (because they are the only options that return the promise from the .then() handler) so the chain will wait for that promise to resolve.

2. In options 1, 2 and 4, if doSomethingElse() throws a synchronous exception, then the promise chain will bee rejected with the exception as the reason. In option 3, the function is executed outside the promise chain so if it throws a synchronous exception, it will throw to a higher scope (similar to if doSomething() threw synchronously).

In first case, after resolving promise returned by doSomething(), callback is invoked. Callback function can return another promise (that's why we call promises 'chainable'). In your case, doSomethingElse() is expected to return another promise.

In second case, nothing is returned by callback function, thus next promises in execution chain will be invoked, but with undefined parameter.

In third case, callback function is provided as result of doSomethingElse() function.

And finally, doSomethingElse is expected to be a function, which will be executed after fulfilling first promise.