Top JavaScript Interview Questions [2024]

Various keywords can be used to declare a variable in JavaScript. const and let create block-scoped variables. Additionally, the value of a const variable can't be changed through reassignment once it is created. var creates a function-scoped or a globally-scoped variable, depending on where it is declared. Multiple declarations of var using the same variable are permitted even under the same scope.

Variables declared using the const keyword can not be reassigned. This makes them unsuitable to be used as a loop counter. The remaining options are the let and var keywords. Since var is not block-scoped, it can cause issues if passed into asynchronous functions. An asynchronous function receiving a variable declared using the var keyword can end up referencing the value of that variable at the end of the iteration. This is usually not desired. For that reason, the block-scoped let variables are most suited to be used as a loop counter.

Arrow functions offer a shorter alternative to the traditional function declarations but are limited in some ways. The most significant difference is the lack of bindings to the this keyword, which makes them unsuitable for class methods.

JavaScript is a single-threaded language. It utilizes an event loop to achieve a degree of concurrency where the async events are handled in separate queues and only inserted into the event loop once their results become available.

Callbacks that are passed into the asynchronous functions (such as setTimeout) are not pushed directly into the call stack. Rather, they are enqueued inside the message queue even when the delay for execution is set to 0. This means that the execution of these functions would get delayed until the call stack is empty.

Promises are objects that will return a value or fail sometime in the future. It allows for non-blocking functionality so that your application is not blocked while waiting for asynchronous operations to be completed.

A promise can either be pending, fulfilled or rejected. A fulfilled promise can be resolved with a value or rejected with a reason.

The fetch function allows you to interact with resources using Request and Response objects. It's commonly used for working with resources across the network.

Mutable data types are data types that can have their value changed after getting created. Objects and Arrays are like this. Immutable types can't be altered after their creation. You simply need to create new values to replace the old ones. Strings and Numbers are examples of immutable data types.

First-class functions mean that functions are treated like any other variable in the programming language. As a result, functions can be assigned to variables, passed as arguments to other functions or returned from functions.

A pure function is a function that always returns the same output when given the same input. Additionally, pure functions don't have any side effects. This means that they don't mutate the passed arguments or the program state in any way. Pure functions are easier to reason about as their behaviour is predictable. They're also easier to test since they don't alter the state or the arguments. And they can be more performant because they can easily be parallelized and memorized.

Getting passed by reference is a property of mutable types. It relates to the variable name being a pointer (a reference) to the underlying value that can always get updated. For example, an array passed into a function as an argument can get updated (mutated) inside that function. However, this isn't the case with strings and numbers that are immutable because they're passed by value and can't be updated.

A closure is where an inner function retains access to its parent (outer) function scope, even after the parent function has returned. It's useful for creating private and read-only internal variables since the internals of the parent can only be accessed by this inner function.

Strict mode executes your code with a more restrictive JavaScript variant to guard against some common mistakes. It also makes errors to be more explicit. Issues that would normally silently fail in JavaScript would throw an error in strict mode. It's essentially a safer and recommended environment to develop JavaScript applications.

A constructor function is a function that you can use to create object instances. So you can think of it as a class, a blueprint for objects to be created. Constructor functions in JavaScript are called with the new keyword.

Javascript has a dynamic type system. This means that variables can be assigned and re-assigned any value by default. This is a source of a lot of bugs in JavaScript applications. TypeScript is a build-time type system that helps with this problem. As types are analyzed and sometimes even inferred statically, TypeScript additionally provides tight integration with the IDE by providing auto-complete support. TypeScript code converts to regular JavaScript code so it can run anywhere that JavaScript runs. A disadvantage of TypeScript might be that the syntax can be hard to read and understand initially, introduces new features to learn and can increase the complexity of the build configuration.

There was a time when JQuery was the go-to library for working with JavaScript in the browser. This is not usually the case anymore since the browser incompatibilities that necessitate the usage of jQuery have been largely removed. It would make sense for new projects to avoid using jQuery. However, jQuery can still be relevant when working on an existing project that uses it as a dependency.

CPU-intensive tasks can be problematic for JavaScript since the main execution runs on a single thread. As such, CPU-intensive tasks need to be handled with care to not block the event loop from executing other tasks. Ways to deal with this in the browser could be chunking the slow-running operation into smaller parts so it can be handled in steps, using workers to offload the job to other threads or sending the job to a server so that it can be handled remotely.

JavaScript blocks the DOM when it's executed, which is why it's important to execute it after the page load is completed. You can consider placing your JavaScript file at the bottom of the body element or use the async or defer keywords to delay the parsing and execution of it. If using the async argument, you should be aware that it would still be executed as soon as it is downloaded, so it can still block if downloaded before other resources.

JavaScript is a multi-paradigm programming language. It supports functional programming as functions are treated as first-class objects. You can also do object-oriented programming in JavaScript since it provides support for it through prototypical inheritance model. JavaScript in the browser follows an event-driven model where you execute JavaScript operations as a response to events like user actions.