JavaScript and TypeScript Interview Questions Explained With Real Production Examples
Most explanations of these concepts stop at the definition. Scope gets a rule about hoisting. Closures get a counter function that increments a number nobody would ever ship. The event loop gets a diagram. any versus unknown gets a one-line rule. Generics get a toy example with numbers and strings that never shows up in a real codebase. That's enough to answer a question in an interview. It's not…
JavaScript and TypeScript are popular programming languages used in web development. They have various concepts that are frequently tested in interviews, such as scope, closures, event loop, generics, and more. Understanding these concepts is crucial for writing efficient and maintainable code in real-world applications.
One of the fundamental aspects of JavaScript is variable declaration. There are three ways to declare a variable in JavaScript: var, let, and const. Var is function-scoped and can be redeclared and reassigned freely. Let is block-scoped and can be reassigned but not redeclared in the same scope. Const is also block-scoped but cannot be reassigned after its initial value is set, although the object or array it points to can still be mutated internally.
The recommended practice is to use const by default, let when a value changes, and avoid var altogether.
The scope and reassignability of variables impact the readability and maintainability of code. Properly choosing between const, let, and var helps indicate which values are fixed and which ones can change. Using const for values that don't change enhances code clarity, as it signals to readers that these values are stable and unlikely to be modified.
Closures are another essential concept in JavaScript. A closure is a function that retains access to variables from the scope in which it was created, even after that outer scope has completed execution. A common real-world use case for closures is creating an API client factory function. This function returns an object with methods that can make HTTP requests while retaining access to specific configuration variables.
Each client created through this function maintains its own configuration, even though the factory function has finished executing. This pattern is useful for creating multiple independent clients from a single factory function, each remembering its own settings without shared state.
Asynchronous programming in JavaScript is handled using Promises. Promise.all() is a method that accepts an array of promises and resolves once all of them have resolved, returning their results in the same order. If any promise in the array rejects, Promise.all() rejects immediately, discarding the results of the other promises. This method is suitable when all the promises are dependent on each other and must complete in sequence.
However, Promise.all() may not be the best choice when the promises are independent, and the failure of one should not affect the others. In such cases, Promise.allSettled() is more appropriate. It waits for all promises to settle, whether they are fulfilled or rejected, and returns an array containing the result of each promise.
This method ensures that all results are available, even if some promises fail. For instance, if various components on a web page make independent API requests using Promise.allSettled(), a failure in one request will not prevent the successful results from being processed.
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