How to Synchronously determine a JavaScript Promise’s State
Handling asynchronous operations in JavaScript can be challenging. However, Promises has made this task easier by providing a more elegant and better error management solution. Although Promise-based code is primarily designed to work asynchronously, there may be instances where you need to synchronously determine a Promise’s state for various reasons. In this article, we’ll explore how to do just that.
A Promise represents a value that might not be available yet but will be at some point in the future. It has three states:
- Pending: It is the state when the Promise is created and hasn’t been resolved or rejected yet.
- Fulfilled: The Promise has successfully resolved with a value.
- Rejected: The Promise has encountered an error or exception during its execution of code.
Need for Synchronously Determining a Promise’s State
In most cases, you should rely on the asynchronous nature of Promises to handle your operations. However, there might be scenarios where you need to synchronously check a Promise’s state. Here are a few common use cases:
- Error Handling: You may want to take different actions based on whether a Promise has been rejected or not. Synchronous checking can help you make immediate decisions.
- Conditional Execution: Depending on a Promise’s state, you might want to execute specific code paths synchronously.
- Debugging and Logging: For debugging purposes, you may need to know the state of a Promise at a particular point in your code.
Table of Content
- .then() or .catch()
- Wrap Promises in a synchronous function:
.then() or .catch()
To synchronously determine a JavaScript Promise’s state, you can inspect its properties and methods. In particular, you can check if the Promise has the .then() or .catch() method, as these methods are added to a Promise object when it is created.
Example: In this example, we will see how to synchronously determine a JavaScript Promise’s state by using the .then and .catch methods.
Javascript
const myPromise = new Promise((resolve, reject) => { setTimeout(() => { resolve( "Successfull" ); }, 1000); }); async function checkPromiseState() { try { const result = await myPromise; console.log( "Promise is pending or fulfilled." ); // Handle the fulfilled state here } catch (error) { console.log( "Promise is rejected." ); } } checkPromiseState(); |
Explanation
- In this example, we create a Promise myPromise and check for the existence of the .then() and .catch() methods.
- If the .then() method is present, the Promise is pending or fulfilled.
- If the .catch() method is present, the Promise is rejected.
Output:
Promise is pending or fulfilled.
Wrap Promises in a Synchronous Function
In this case, we can encapsulate the logic of checking a Promise’s state synchronously within a function. This function can return a value or trigger specific actions based on the Promise’s state.
Example: In this example, we will see how to synchronously determine a JavaScript Promise’s state by wrapping the promise inside a function.
Javascript
function checkPromiseState(promise) { return promise .then(() => "fulfilled" ) . catch (() => "rejected" ); } const myPromise = new Promise((resolve, reject) => { setTimeout(() => { resolve( "Success!" ); }, 1000); }); checkPromiseState(myPromise) .then(state => { console.log(`Promise state: ${state}`); }); |
Explanation
- The checkPromiseState function encapsulates a Promise, resolving to “fulfilled” on success and “rejected” on error.
- An example Promise, myPromise, simulates an asynchronous task using setTimeout.
- The code invokes checkPromiseState on myPromise and logs the resulting state: “fulfilled” or “rejected,” indicating the Promise’s resolution.
Output:
Promise state: fulfilled
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