Promises & Callbacks

Preview — 3 of 10 questions

What best describes asynchronous code in JavaScript?

javascript

console.log("Start");

setTimeout(() => {
  console.log("Timer done"); // runs later
}, 1000);

console.log("End"); // runs immediately — doesn't wait

// Output:
// Start
// End
// Timer done (after 1 second)

ACode that runs only in a separate thread

BCode that blocks execution until it completes

CCode that starts an operation and allows other code to continue running while waiting for the result

DCode that only works in Node.js, not in browsers

What is the output of this code?

javascript

console.log("A");
setTimeout(() => console.log("B"), 0);
console.log("C");

AA B C

BA C B

CB A C

DC A B

What best describes a callback function?

javascript

function greet(name, callback) {
  console.log("Hello, " + name);
  callback(); // calls the callback after greeting
}

function sayBye() {
  console.log("Goodbye!");
}

greet("Alice", sayBye);
// Output:
// Hello, Alice
// Goodbye!

AA function that runs immediately when defined

BA function that is passed as an argument to another function to be called at a later time

CA function that only works with setTimeout

DA function that returns another function

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