# Exceptions in promise chain in JavaScript

Suppose for some reason our promise will end with an error:

`let promise = new Promise(function(resolve, reject) { setTimeout(function() { reject('error'); }, 3000); });`

In this case, the execution of the code will immediately jump to `then`, which has an error handler function, or to the first `catch`, whichever comes first.

Here is an example of the first situation:

`promise.then( function(result) { return result + '1'; } ).then( function(result) { return result + '2'; }, function(error) { // an execution jumps to here } ).then( function(result) { console.log(result); } );`

Here is an example of the second situation:

`promise.then( function(result) { return result + '1'; } ).then( function(result) { return result + '2'; } ).catch( function(error) { // an execution jumps to here } ).then( function(result) { console.log(result); } );`

The handler function has two options: if it handled the exception, it can return a result via `return` and execution will continue further down the chain. If it didn't cope with the error, then it can either return nothing, or throw an exception via `throw`. In this case, execution will jump to the next error hook (at `then` or `catch`, whichever comes first).

As a rule, all chain errors are caught in one place: `catch` is placed at the end of the chain:

`promise.then( function(result) { return result + '1'; } ).then( function(result) { return result + '2'; } ).catch( function(error) { // we get here in case of an error } );`

In this case, an exception can occur in the promise itself, or thrown via `throw` in any link in the chain:

`promise.then( function(result) { return result + '1'; } ).then( function(result) { if (everythingIsFine) { return result + '2'; } else { throw new Error('an error'); // we go to the nearest interceptor } } ) .then( function(result) { return result + '3'; } ).catch( function(error) { // the nearest interceptor } );`

Keep in mind that `catch` is needed specifically for diagnosing an error: is it solvable or not. If the error is solvable, then `catch` must pass its solution to the next `then`. And if it isn't solvable (or the given `catch` simply doesn't know how to resolve it), then we must either return nothing or throw an exception:

`promise.then( function(result) { return result + '1'; } ).then( function(result) { return result + '2'; } ).catch( function(error) { if (errorIsSolvable) { return 'data'; // we send it to the next then } else { // we return nothing or throw an exception } } ).then( function(result) { // we solve an error here } );`
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