Javascript Promise Interview Questions
Clarifying Your Knowledge of JavaScript Promises: Common Interview Questions
JavaScript Promise Interview Questions typically revolve around the concept of Promises in JavaScript, which is a way to handle asynchronous operations in a more organized and predictable manner. These questions may cover topics such as the basic syntax of Promises, common methods used with Promises, error handling, and chaining multiple promises. Interviewers may also ask about the advantages and disadvantages of using Promises, as well as how Promises differ from callbacks. Overall, these questions aim to assess a candidate's understanding and proficiency in using Promises in JavaScript.
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1 - What is a promise in JavaScript?
A promise is an object that represents the eventual completion or failure of an asynchronous operation. It allows us to handle asynchronous code in a more organized and readable manner, by providing a structure for handling success or failure cases.
2) How do you create a promise in JavaScript?
To create a promise, we use the Promise constructor and pass in a function with two parameters, usually called “resolve” and “reject”. Inside this function, we define the asynchronous operation and call the “resolve” function when it is successful, or the “reject” function when it fails.
3) What is the difference between a promise and a callback?
Both promises and callbacks are ways to handle asynchronous operations. However, promises provide a more organized and readable way to handle callbacks, as well as the ability to chain multiple asynchronous operations together.
4) What are the three states of a promise?
A promise can have three states:
Pending: Initial state, neither fulfilled nor rejected.
Fulfilled: The asynchronous operation has completed successfully.
Rejected: The asynchronous operation has failed.
5) How do you handle errors in promises?
Promise objects have a “.catch()” method, which allows us to handle any errors that occur during the asynchronous operation. This helps to prevent the entire application from crashing if one operation fails.
6) What is a promise chain?
A promise chain is when we chain multiple asynchronous operations together, using the “.then()” method to specify what should happen after each operation completes. This allows us to handle complex asynchronous code in a more organized and readable way.
7) What is the purpose of the “resolve” and “reject” functions?
The “resolve” function is used to fulfill a promise and return the result of a successful asynchronous operation. The “reject” function is used to reject a promise and return the reason for the failure of the asynchronous operation.
8) How do you handle multiple promises in parallel?
We use the “Promise.all()” method to handle multiple promises in parallel. This method takes in an array of promises and returns a new promise that resolves when all the promises in the array have been fulfilled, or rejects if any of the promises are rejected.
9) Can a promise be resolved or rejected more than once?
No, a promise can only be resolved or rejected once. After it has been resolved or rejected, any subsequent calls to the “resolve” or “reject” functions will be ignored.
10) What is the purpose of the “.finally()” method?
The “.finally()” method is used to perform an action after a promise has been resolved or rejected, regardless of the outcome. This can be useful for performing cleanup operations or displaying loading messages.
11) How do you convert callback based functions into promise based functions?
We can use the “util.promisify()” method in Node.js or use wrappers to convert callback based functions into promise based functions. We can also use libraries like “axios” or “request promise” to make HTTP requests as promises.
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12) How do you handle errors in promise chains?
We can use multiple “.catch()” statements in a promise chain to handle different types of errors. We can also use the “finally” method at the end of the chain to perform any necessary cleanup operations.
13) What is the purpose of the “Promise.race()” method?
The “Promise.race()” method is used to handle a group of promises in a race condition. It takes in an array of promises and returns a new promise that is fulfilled or rejected as soon as one of the promises in the array is fulfilled or rejected.
14) How do you handle asynchronous operations in a synchronous function?
We can use the “async/await” syntax to handle asynchronous operations in a synchronous function. The “async” keyword is used in the function declaration to tell JavaScript that the function will be asynchronous, and the “await” keyword is used to pause the execution of the function until the asynchronous operation has completed.
15) What is the difference between a promise and an observable?
A promise is only capable of handling a single asynchronous operation, whereas an observable can handle multiple operations and emit multiple values over time. Observables also provide more features like the ability to cancel the operation, retry, or handle errors in a more granular way.
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