When to Use Promise.reject vs. Throw in JavaScript Promises?

JavaScript Promises: The Enigma of Reject vs. Throw

When working with JavaScript promises, developers often face a dilemma: should they use Promise.reject or simply throw an error? While both methods serve a similar purpose, confusion lingers regarding their differences and potential advantages.

Exploring the Similarities

Ultimately, there is no inherent advantage in using Promise.reject over throwing an error or vice versa. Both mechanisms set the promise to the rejected state and trigger the execution of the .catch or catch() handler.

Unveiling a Subtle Distinction

However, a specific case exists where throwing an error falls short: asynchronous callbacks outside of promise callbacks. In these situations, Promise.reject is the only option to make the rejected state known to the promise chain.

Consider the following example:

<code class="javascript">new Promise(function() {
  setTimeout(function() {
    throw 'or nah';
    // Using Promise.reject('or nah') also won't work in this case
  }, 1000);
}).catch(function(e) {
  console.log(e); // doesn't happen
});</code>

In this scenario, the error thrown inside the setTimeout callback will not be caught by the .catch handler because it is not executed within a promise callback. To handle this type of situation effectively, Promise.reject must be used within the asynchronous callback.

Choosing the Best Practice

In general, either Promise.reject or throwing an error can be used to set a promise to the rejected state. However, when working with asynchronous callbacks outside of promise callbacks, Promise.reject becomes the only viable option.

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