When to Use Promise.reject Instead of Throw?

Promise.reject vs. throw: Unveiling the Differences

A common dilemma in JavaScript development is understanding the distinction between Promise.reject and throwing an error. While both strategies can be used to handle errors, a subtle difference emerges in specific scenarios.

In the context of promise callbacks, using throw effectively signals an error, and the following catch handler will capture it. However, when working outside of promise callbacks, the catch handler will not be invoked if throw is employed. To remedy this, Promise.reject must be used instead.

Consider the following code:

Using Promise.reject:

<code class="javascript">return asyncIsPermitted()
    .then(function(result) {
        if (result === true) {
            return true;
        }
        else {
            return Promise.reject(new PermissionDenied());
        }
    });</code>

Using throw:

<code class="javascript">return asyncIsPermitted()
    .then(function(result) {
        if (result === true) {
            return true;
        }
        else {
            throw new PermissionDenied();
        }
    });</code>

In the above example, the throw approach would trigger the catch handler in any promise callback. However, if asyncIsPermitted() is not a promise and is instead a function that performs asynchronous processing (using setTimeout or similar), the throw would not be handled by the catch. In such cases, Promise.reject should be used to ensure that the error is captured.

Therefore, while throw may be preferable due to its brevity, Promise.reject offers more flexibility and should be used when necessary to ensure error handling in non-promise callback contexts.

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