Programming Languages & Core Development Topics
Programming languages, development fundamentals, coding concepts, and core data structures. Includes syntax, algorithms, memory management at a programming level, asynchronous patterns, and concurrency primitives. Also covers core data manipulation concepts like hashing, collections, error handling, and DOM manipulation for web development. Excludes tool-specific proficiency (see 'Tools, Frameworks & Implementation Proficiency').
JavaScript Scope, Closures and Context
Comprehensive mastery of JavaScript scoping rules, closure mechanics, and execution context including the this binding. Candidates should understand lexical scope and the scope chain, differences between function scope and block scope, and the behaviors and hoisting implications of var, let, and const. Know how closures capture and retain variables from outer scopes, how closures enable function factories and data privacy, and common closure patterns such as module patterns, factory functions, immediately invoked function expressions, currying, and decorators. Be able to apply closures safely in asynchronous code including timers, callbacks, and promises, and reason about pitfalls such as closures in loops, accidental retention of large objects, memory and garbage collection consequences, hoisting related bugs, and performance tradeoffs. Understand execution context and the this binding, including how call site determines this in global calls, method calls, constructor calls, and event handlers, how arrow functions use lexical this, and how to use call, apply, and bind to control context. Interview tasks typically include reading and explaining existing closures, identifying and fixing closure related bugs, implementing small patterns or utilities that rely on closures and context, and discussing debugging and optimization strategies. This also covers framework relevant patterns such as custom hooks in component based frameworks where closures and context interact with lifecycle and state.
Debugging and Code Optimization
Practical debugging skills and techniques for improving code performance and complexity. Topics include tracing and reproducing bugs, stepping through execution, reasoning about time and space complexity, refactoring for performance, and applying algorithmic optimizations. Candidates should be able to demonstrate logical debugging approaches and make safe, measurable performance improvements to working code.
Error Handling and Defensive Programming
Covers designing and implementing defensive, fault tolerant code and system behaviors to prevent and mitigate production failures. Topics include input validation and sanitization, null and missing data handling, overflow and boundary protections, exception handling and propagation patterns, clear error reporting and structured logging for observability, graceful degradation and fallback strategies, retry and backoff policies and idempotency for safe retries. Also address concurrency and synchronization concerns, resource and memory management to avoid exhaustion, security related input checks, and how to document and escalate residual risks. Candidates should discuss pragmatic trade offs between robustness and complexity, show concrete defensive checks and assertions, and describe test strategies for error paths including unit tests and integration tests and how monitoring and operational responses tie into robustness.
DOM Manipulation and Browser APIs
Practical knowledge of the Document Object Model and browser application programming interfaces used to implement interactive user interfaces. Topics include querying and traversing elements, modifying classes and attributes, creating and removing elements, event handling and delegation, event propagation and capturing, safe insertion to avoid cross site scripting, efficient DOM updates to minimize reflows and repaints, layout thrashing and mitigation techniques, use of Fetch and storage APIs, cross browser considerations, and accessibility basics. Interviewers may ask for short implementations and discussion of performance and security trade offs.
Asynchronous JavaScript and Event Loop
Comprehensive understanding of asynchronous programming in JavaScript including callback functions and the pitfalls of callback nesting, Promises including their lifecycle and state transitions, promise chaining, and common utility methods such as Promise.all, Promise.race, and Promise.allSettled. Knowledge of async and await as syntactic sugar over Promises, proper error handling patterns using try catch and promise catch, and avoiding unhandled rejections. Practical skills for working with multiple concurrent operations such as coordinating parallel requests, implementing retry logic, rate limiting, throttling and debouncing of async calls, and handling race conditions. Deep understanding of the JavaScript event loop model including the call stack, task queue, microtask queue, the ordering of macrotasks versus microtasks, how timers and promise callbacks are scheduled, and how this affects concurrency and ordering. Familiarity with implementing a simplified Promise from first principles to demonstrate internal behavior, diagnosing timing and ordering bugs, and designing solutions for real world asynchronous flows and performance considerations.
Objects and Object Oriented Basics
Creating objects, accessing and modifying properties, object methods, destructuring, spread operator, Object methods (Object.keys(), Object.values(), Object.entries()). Basic understanding of prototypes.
Array Methods and Object Manipulation
Focuses on language level array operations and common data transformation patterns together with manipulating objects or records. Includes array methods such as map, filter, reduce, sort, find and for each and when to use each method, stable and unstable sorting considerations, and performance implications of chained operations. Also covers object property manipulation, mapping between arrays and objects, merging and immutability patterns, and common transformation tasks in data processing pipelines. Candidates should understand algorithmic complexity of these operations, side effects versus pure functions, and practical usage in coding interviews and production code.
Variables, Data Types, and Scope
Understanding var, let, const, their differences, and scope (global, function, block scope). Knowing hoisting, temporal dead zone, and best practices for variable declaration.
Functions and Callbacks
Covers the full set of core function concepts used in JavaScript and similar languages: functions as first class objects, function declarations versus function expressions, arrow functions, parameters and return values, and passing functions as arguments as callbacks. Includes higher order functions that accept or return functions, patterns for customizing behavior with callbacks, and common uses such as event handlers, iterator callbacks, and asynchronous callbacks. Also addresses related concepts often assessed together such as closures, function scope and hoisting, and the basic function execution context including the this keyword. Interviewers may probe understanding through examples, small code transformations, explaining behavior, and using these concepts to solve practical problems.