JavaScript Interview Questions

Introduction

• The video covers five JavaScript questions commonly asked in interviews.
• Questions will test understanding of JavaScript fundamentals, specifically around let, var, and other concepts.

Question 1: Difference Between let and var

• Purpose: To check understanding of block scope versus function scope.
• Explanation:
  • let creates block-scoped variables.
  • Variables defined with let cannot be accessed outside their block.
• Output Behavior:
  • If let a = 1 and let b = 2 are defined in a block and accessed outside, it will throw a ReferenceError.
  • Only the outputs from within the block will be shown before the error occurs.

Question 2: Hoisting Behavior of var and let

• Question: What happens when we console.log a variable before it is assigned?
• Output:
  • For var, it returns undefined because var is hoisted.
  • For let, it throws a ReferenceError due to the Temporal Dead Zone (TDZ).
• Example with name variable:
  • If var name = '' is declared, console.log(name) outputs an empty string, not undefined.

Question 3: Variable Shadowing

• Context: Understanding how local and global scopes interact.
• Example:
  • A global var x = 20 and a local var x = 10 will cause the local variable to shadow the global one.
  • Accessing x inside the function returns undefined because the function creates its variable before the assignment.
• Key Point: If a local variable exists, the function will not access the global variable.

Question 4: Closures

• Definition: A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope.
• Example:
  • Returning an inner function from an outer function allows access to variables from the outer function.
• Verification: Using console.dir can show how variables are bundled with the inner function.

Question 5: Event Loop and Asynchronous Behavior

• Main Idea: Understand the execution order of synchronous vs asynchronous code.
• Output Sequence: The order of logs will be start, end, and then timeout.
• Explanation:
  • console.log statements are synchronous and execute in order.
  • setTimeout is asynchronous and does not execute until the call stack is empty, even if the timeout is set to 0 milliseconds.
• Implication: Event loop processes callbacks only after completing the synchronous code execution.

Conclusion

• Emphasize understanding the fundamentals of JavaScript rather than memorizing answers.
• Encouraged to explore more about JavaScript basics and asynchronous behavior.