Do you know Promises in JS ? (Basic)

Introduction

Hey forks !
In this blog post, we are going to explore 10 tricky JavaScript questions that test your understanding of Promises and asynchronous behaviour of Javascript.

What are Promises and how to handle it in JS ?

• JavaScript Promises are a powerful tool for handling asynchronous operations.
• They represent a value that may be available now, in the future, or never.

• Promises simplify complex asynchronous code by providing a more readable and maintainable structure. With methods like .then(), .catch(), and .finally(), Promises allow you to chain operations and handle errors gracefully.

• Building on Promises, async and await are syntactic sugar that make asynchronous code look and behave more like synchronous code.

• The async keyword declares an asynchronous function, which returns a Promise, while the await keyword pauses the execution until the Promise is resolved.

• This modern approach simplifies error handling and reduces the need for complex Promise chains, making your code cleaner and more maintainable.

• Understanding Promises and async/await is essential for writing modern, efficient JavaScript code, especially when dealing with APIs, timers, or any asynchronous tasks.

Now let's start solving some basic promise related output based questions which might help you to crack the interviews.

Example 1:

Code

let p = new Promise((resolve, reject) => {
  resolve("Hello");
});
p.then(value => console.log(value));
console.log("World");

output ✅:

World
Hello

Explanation 💡:

• The console.log("World") executes immediately.
• The Promise resolves and logs "Hello" after the synchronous code has finished executing.

---

Example 2:

Code

let p = new Promise((resolve, reject) => {
  reject("Error");
});
p.catch(error => console.log(error));
console.log("After promise");

output ✅:

After promise
Error

Explanation 💡:

• The console.log("After promise") executes immediately.
• The Promise is rejected and logs "Error" after the synchronous code has finished executing.

---

Example 3:

Code

let p = new Promise((resolve, reject) => {
  resolve("First");
});
p.then(value => {
  console.log(value);
  return "Second";
}).then(value => console.log(value));
console.log("Third");

output ✅:

Third
First
Second

Explanation 💡:

• The console.log("Third") executes immediately.
• The Promise resolves and logs "First", then the chained then logs "Second".

---

Example 4:

Code

let p = Promise.resolve("Success");
p.then(value => {
  console.log(value);
  throw new Error("Fail");
}).catch(error => console.log(error.message));
console.log("Done");

output ✅:

Done
Success
Fail

Explanation 💡:

• The console.log("Done") executes immediately.
• The Promise resolves and logs "Success", then the error is caught and "Fail" is logged.

---

Example 5:

Code

let p = new Promise((resolve, reject) => {
  setTimeout(() => resolve("Timeout"), 1000);
});
p.then(value => console.log(value));
console.log("Immediate");

output ✅:

Immediate
Timeout

Explanation 💡:

• The console.log("Immediate") executes immediately.
• The Promise resolves after 1 second and logs "Timeout".

---

Example 6:

Code

async function asyncFunc() {
  return "Async";
}
asyncFunc().then(value => console.log(value));
console.log("Function");

output ✅:

Function
Async

Explanation 💡:

• The console.log("Function") executes immediately.
• The asyncFunc resolves and logs "Async" after the synchronous code has finished executing.

---

Example 7:

Code

let p = new Promise((resolve, reject) => {
  resolve("Resolved");
});
p.finally(() => console.log("Finally"))
 .then(value => console.log(value));
console.log("End");

output ✅:

End
Finally
Resolved

Explanation 💡:

• The console.log("End") executes immediately.
• The Promise finally logs "Finally" and then logs "Resolved".

---

Example 8:

Code

async function asyncFunc() {
  return new Promise((resolve, reject) => {
    setTimeout(() => resolve("Done"), 500);
  });
}
asyncFunc().then(value => console.log(value));
console.log("Start");

output ✅:

Start
Done

Explanation 💡:
The console.log("Start") executes immediately.

The asyncFunc resolves and logs "Done" after 500ms.

Example 9:

Code

let p = Promise.resolve("First");
p.then(value => {
  console.log(value);
  return Promise.resolve("Second");
}).then(value => console.log(value));
console.log("Third");

output ✅:

Third
First
Second

Explanation 💡:

• The console.log("Third") executes immediately.
• The Promise resolves and logs "First", then the chained then logs "Second".

---

Example 10:

Code

async function asyncFunc() {
  console.log("Inside function");
  return "Async";
}
asyncFunc().then(value => console.log(value));
console.log("Outside function");

output ✅:

Inside function
Outside function
Async

Explanation 💡:

• The asyncFunc logs "Inside function" immediately.
• The console.log("Outside function") executes immediately.
• The asyncFunc resolves and logs "Async" after the synchronous code has finished executing.

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That’s all from my side, keep practising forks 🔥!!!
🫡See you in next blog.

Happy Coding 💻...