Why Does The Promise Constructor Require A Function That Calls Resolve When Complete But Then Does Not It Returns A Value Instead

When working with promises in JavaScript, you may come across a common question: why does the Promise constructor require a function that calls `resolve` when complete but then does not it return a value instead? This may seem a bit confusing at first, but once you understand the inner workings of promises, it will become clear why this is the case.

Let's break it down step by step. When you create a new promise using the Promise constructor, you need to pass in a function that takes two parameters: `resolve` and `reject`. Inside this function, you perform some asynchronous task, such as fetching data from an API or reading a file. Once this task is complete, you call the `resolve` function to signal that the promise has been fulfilled successfully.

The reason why the promise constructor does not return a value directly is that promises are designed to handle asynchronous operations. When you call `resolve` inside the promise executor function, you are essentially saying, "I have completed the asynchronous task successfully." At this point, the promise transitions from a pending state to a fulfilled state. If there is an error during the task, you can call the `reject` function to indicate a failure.

So, if the promise constructor were to return a value directly, it would go against the asynchronous nature of promises. Instead, the value that you want to return from the asynchronous task should be passed to the `resolve` function as an argument. This value will be made available to the `then` handler that you attach to the promise to handle the resolved value.

Here's an example to illustrate this concept:

Javascript

const fetchData = new Promise((resolve, reject) =&gt; {
  // Simulate fetching data asynchronously
  setTimeout(() =&gt; {
    const data = { id: 1, name: 'John Doe' };
    resolve(data); // Pass the data to the resolve function
  }, 2000);
});

fetchData.then((data) =&gt; {
  console.log(data); // Output: { id: 1, name: 'John Doe' }
});

In this example, we create a new promise called `fetchData` that simulates fetching data after a delay of 2 seconds. Once the data is retrieved successfully, we call the `resolve` function with the data object. The `then` handler attached to the promise `fetchData` will then receive the resolved data and log it to the console.

By understanding why the promise constructor requires a function that calls `resolve` when complete but does not return a value directly, you can leverage the power of promises to manage asynchronous operations effectively in your JavaScript applications. Remember, promises are all about handling asynchronous tasks in a clear and concise manner, and the separation of concerns between the promise constructor and the resolved value plays a crucial role in achieving this.

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