Front
What is a closure in JavaScript?
Back
A function bundled with its lexical environment.
Allows an inner function to access variables from an outer scope even after the outer function has returned.
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JavaScript Interview Prep - 50 Questions
Ace your JavaScript interviews with 50 essential questions covering closures, promises, prototypes, and modern ES6+ features.
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Preview
Front
What is lexical scoping?
Back
Variable access determined by the physical location in source code.
Functions are executed using the scope chain that was in effect when they were defined.
Front
How do closures enable private variables?
Back
By returning an object with methods that access local variables.
Example: function counter() { let count = 0; return () => ++count; }
Front
Why does using 'var' in a loop often cause closure issues?
Back
'var' is function-scoped, not block-scoped.
All iterations share the same variable instance; use 'let' to create a new binding for every loop iteration.
Front
Can closures cause memory leaks?
Back
Yes, if they hold onto large objects unnecessarily.
The garbage collector cannot free memory for variables still referenced by an active closure.
Front
What is function currying?
Back
Transforming a function with multiple arguments into a sequence of nesting functions.
Example: const add = a => b => a + b; add(1)(2); // 3
Front
How is memoization implemented using closures?
Back
By storing a cache object within the outer function's scope.
The inner function checks the cache before performing expensive computations.
Front
What is the scope chain in a closure?
Back
The hierarchy of Local, Outer, and Global scopes.
JavaScript searches for variables starting from the innermost scope outward.
Front
What is a Promise in JavaScript?
Back
An object representing the eventual completion or failure of an asynchronous operation.
It acts as a placeholder for a value that is not yet known.
Front
What are the three states of a Promise?
Back
Pending, Fulfilled, and Rejected.
A promise starts as pending and settles into either fulfilled (success) or rejected (error).
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