--- title: "JavaScript: Language Fundamentals & Modern Features" description: "Test your knowledge of JavaScript fundamentals with this comprehensive quiz covering variables, data types, functions, promises, DOM manipulation, ES6 features, closures, and modern JavaScript programming best practices." author: "Mohammad Abu Mattar" canonical: https://mkabumattar.com/quizzes/post/javascript-fundamentals-quiz --- # JavaScript: Language Fundamentals & Modern Features Welcome to the JavaScript Basics Quiz! This quiz is designed to test your understanding of fundamental JavaScript concepts, syntax, and modern programming best practices. Each question is multiple-choice, and you'll find hints to help you along the way. Good luck! ## Questions ### 1. Which keyword is used to declare a block-scoped variable that can be reassigned? - `var` - The "var" keyword is function-scoped rather than block-scoped and is subject to hoisting. - **`let`** ✅ - The "let" keyword provides block-scoping and permits the variable to be reassigned later. - `const` - While block-scoped, "const" creates a constant reference that prevents value reassignment. - `set` - The "set" keyword is used to define property setters within objects, not to declare variables. **Hint:** Introduced in ES6 to replace "var". ### 2. What is the result of "5" + 2 in JavaScript? - `7` - This result would require both operands to be numeric types without string coercion. - **`"52"`** ✅ - JavaScript coerces the number into a string and concatenates it with the existing string. - `NaN` - NaN occurs when a mathematical operation fails, but addition involving a string is valid. - `undefined` - An expression involving a valid string and number will resolve to a value rather than undefined. **Hint:** JavaScript prioritizes string concatenation with the + operator. ### 3. How do you create a function using the arrow syntax? - `function my_func => {}` - Arrow functions do not use the "function" keyword as part of their syntax. - **`const my_func = () => {}`** ✅ - This syntax defines an anonymous function and assigns it to a constant variable name. - `let my_func = func() {}` - The "func" keyword is not used for function declaration in standard JavaScript. - `my_func() -> { return }` - This syntax is used in languages like Rust or Swift but is invalid for JavaScript arrows. **Hint:** Uses the "fat arrow" symbol =>. ### 4. What does "===" check that "==" does not? - The scope of the variables - Scope is determined by declaration keywords, not by comparison operators. - **Both the value and the type** ✅ - Strict equality (===) ensures no type coercion occurs during the comparison process. - Memory address consistency - While objects are compared by reference, strict equality for primitives checks value and type. - Function execution status - Operators compare data states rather than the execution context of surrounding functions. **Hint:** Think about data types. ### 5. Which method is used to write text directly into the browser console? - `print.log()` - The global print() function is used to trigger the browser printer dialog. - **`console.log()`** ✅ - The console object provides the log() method for outputting debug information to the terminal. - `debug.write()` - Standard JavaScript uses the console object rather than a global debug object for logging. - `terminal.out()` - Terminal output in the browser is standardized through the console API methods. **Hint:** Starts with "c". ### 6. What is the purpose of "document.querySelector()" - Modifying the page window - Window modifications are handled via the window object, not the document selector. - **Selecting an element by CSS** ✅ - This method returns the first element that matches a specified CSS selector string. - Creating new script tags - Creating elements is done via document.createElement() rather than a query selector. - Refreshing the document - Page refreshing is controlled by the location.reload() method in the browser. **Hint:** It interacts with the HTML page. ### 7. Which of these is NOT a valid way to declare a variable in modern JS? - `let` - The "let" keyword is the standard method for declaring variables with block scope. - **`set`** ✅ - The "set" keyword is used to define object setters and is not a variable declaration tool. - `var` - While legacy and generally avoided, "var" remains a valid keyword for function-scoped variables. - `const` - The "const" keyword is used to declare variables that cannot be reassigned after initialization. **Hint:** One of these is from a different language. ### 8. What is a "Promise" in JavaScript? - A syntax for error suppression - Errors are handled within promises using .catch(), but they are not for general suppression. - **An object for async task state** ✅ - A Promise represents the eventual result of an asynchronous operation and its resulting value. - A lock for immutable variables - Variable immutability is handled by the "const" keyword or the Object.freeze() method. - A server-side validation rule - Promises are a client-side or environment-side control flow tool, not a validation protocol. **Hint:** Used for asynchronous operations. ### 9. Which array method creates a new array by performing a function on every element? - `forEach()` - The forEach() method iterates over items but does not return a new array result. - **`map()`** ✅ - The map() method transforms each element and returns a new array containing the results. - `filter()` - The filter() method creates a subset of an array based on a condition without transforming values. - `push()` - The push() method modifies the original array by adding an element to the end of it. **Hint:** It "maps" values to new values. ### 10. What does the "this" keyword refer to in a standard object method? - The root execution context - In a method, "this" is bound to the object instance rather than the root window context. - **The object owning the method** ✅ - The "this" keyword allows a method to access other properties and methods on the same object. - The internal prototype chain - While "this" can access inherited properties, it refers to the object instance itself. - The parent function caller - JavaScript does not use "this" to refer to the function that called the current method. **Hint:** It refers to the owner. ### 11. How do you convert a JSON string into a JavaScript object? - `JSON.stringify()` - The stringify method converts a JavaScript object into a JSON-formatted string. - **`JSON.parse()`** ✅ - The parse method takes a JSON string and converts it into a native JavaScript object. - `Object.convert()` - JavaScript does not have a global convert method on the Object prototype for JSON. - `String.toJSON()` - Standard strings do not have an internal method for converting themselves to JSON objects. **Hint:** You "parse" the string. ### 12. What is "Hoisting" in JavaScript? - Uploading files to a server - Transferring files to a server is known as deployment or uploading, not hoisting. - **Moving declarations to the top** ✅ - Hoisting is the default behavior of moving declarations to the top of the current scope. - Increasing memory allocation - Memory management is handled by the garbage collector and engine, not by hoisting logic. - Optimizing function calls - While related to execution phase, hoisting is about scope visibility rather than optimization. **Hint:** Variables and functions being moved to the top. ### 13. Which operator is used to spread the elements of an array into another array? - **`...`** ✅ - The spread operator (...) expands an iterable into individual elements within a new array. - `***` - Triple asterisks are not valid operators within the JavaScript language specification. - `&&&` - Logical AND uses two symbols (&&); three symbols are not a recognized JavaScript operator. - `!!!` - While single and double exclamation marks are used for coercion, triple marks have no special function. **Hint:** Three dots. ### 14. What is an "anonymous function"? - A function with hidden code - All JavaScript function logic is visible; anonymity refers only to the identifier name. - **A function without a name** ✅ - Anonymous functions are defined without a title and are often used as immediate callbacks. - A function that only runs once - Any function can be limited to one run, but anonymity does not impose this restriction. - A function for private scopes - Privacy is achieved via closures or private class members, not via anonymous naming. **Hint:** A function with no name. ### 15. Which of these is a "falsy" value in JavaScript? - `"0"` - A non-empty string is considered truthy in JavaScript, even if it contains the character "0". - **`0`** ✅ - The numeric value 0 is one of the standard falsy values defined in the language engine. - `[]` - Empty arrays are objects in JavaScript and are evaluated as truthy in boolean contexts. - `{}` - Empty objects are considered truthy; they must be checked for keys to determine if they are "empty". **Hint:** Something that behaves like False in an "if" statement. ### 16. What is the purpose of the "async" keyword? - To increase processing speed - The async keyword manages concurrency and waiting without increasing the raw execution speed. - **To return a Promise object** ✅ - Functions marked with async always return a Promise and allow the use of the await keyword. - To encrypt server requests - Encryption is handled by networking protocols like HTTPS, not by the async keyword. - To bypass the event loop - Async functions operate within the event loop to manage non-blocking task execution. **Hint:** It prepends a function definition. ### 17. How do you check the length of an array "myArr"? - `myArr.count()` - The count() method is common in C# or Swift, but it is not a native JavaScript array method. - **`myArr.length`** ✅ - The length property returns the number of elements currently stored in the array. - `myArr.size` - The size property is used for Map and Set collections, while arrays use length. - `len(myArr)` - The len() function is a Python built-in and is not part of the JavaScript global scope. **Hint:** Property name. ### 18. What does "DOM" stand for? - Data Object Model - While the DOM represents data, the official acronym uses "Document" for the first letter. - **Document Object Model** ✅ - The DOM is the structural representation of the HTML document as a tree of objects. - Digital Ordinary Map - This is an incorrect expansion; the term refers specifically to the document structure. - Direct Output Mode - This refers to rendering or buffer modes rather than the document object structure. **Hint:** The structure of the webpage. ### 19. Which keyword is used to stop a loop immediately? - `stop` - JavaScript does not recognize "stop" as a valid keyword for controlling loop iteration. - **`break`** ✅ - The "break" keyword terminates the current loop and resumes execution after the loop block. - `exit` - The "exit" command is typically used in shell scripting or to terminate a full process. - `halt` - While "halt" implies stopping, it is not a reserved keyword for loop control in JavaScript. **Hint:** Same as in Java or Python. ### 20. What is the "event bubble"? - A visual tooltip element - Bubbling refers to the internal propagation of events, not to a user interface component. - **Event propagation upwards** ✅ - Event bubbling describes how an event moves from the target element up through its parents. - A browser memory error - Bubbling is a standard architectural feature of the DOM event model, not a memory fault. - A recursive function call - While it involves a sequence of triggers, bubbling is handled by the browser event engine. **Hint:** How events travel up the DOM. ### 21. Which symbol is used for template literals? - `Double quotes (" ")` - Double quotes create standard strings and do not support native expression interpolation. - **`Backticks (` `)`** ✅ - Backticks define template literals, allowing for multi-line strings and embedded variables. - `Single quotes (' ')` - Single quotes are functionally similar to double quotes and lack template literal features. - `Tildes (~ ~)` - The tilde is a bitwise NOT operator and is not used to encapsulate string literals. **Hint:** The key next to the "1". ### 22. What is the purpose of "use strict"? - To encrypt source code - Strict mode focuses on code quality and error prevention rather than source security. - **To enforce cleaner code** ✅ - Enabling strict mode catches common silent errors and prevents the use of unsafe features. - To allow legacy support - Strict mode actually disables some legacy features to promote modern coding standards. - To minify script files - Minification is a build-step process, while "use strict" is a runtime directive for the engine. **Hint:** It makes JS less "loose". ### 23. What is "null" in JavaScript? - An undeclared variable - Undeclared variables result in a ReferenceError, whereas "null" is an assigned value. - **Intentional empty value** ✅ - The "null" value is used by developers to explicitly signal that a variable has no object value. - The numeric value of 0 - While 0 is falsy, "null" is a distinct primitive type representing the absence of an object. - A boolean false state - Although falsy in conditional checks, "null" is not the same as the boolean "false". **Hint:** An intentional absence. ### 24. Which method adds an item to the BEGINNING of an array? - `push()` - The push() method adds elements to the end of the array, increasing its length. - **`unshift()`** ✅ - The unshift() method adds one or more elements to the start of an array and shifts others. - `shift()` - The shift() method removes the first element from an array rather than adding a new one. - `prepend()` - While prepend() exists for DOM nodes, standard JavaScript arrays use the unshift() method. **Hint:** The opposite of push. ### 25. What does "NaN" stand for? - Null and Negative - Null and Negative are distinct numeric states, while NaN represents an undefined result. - **Not a Number** ✅ - NaN is a numeric data type value that represents an undefined or unrepresentable mathematical result. - No active Network - Networking status is not related to the NaN numeric type in JavaScript core. - New assigned Node - The DOM uses Node objects, but NaN is strictly related to numeric operations. **Hint:** Not A... ### 26. Which function is used to execute a block of code after a specified delay? - `setInterval()` - The setInterval() function repeats the code execution indefinitely at a fixed time interval. - **`setTimeout()`** ✅ - The setTimeout() function executes the provided code once after the specified delay in milliseconds. - `wait()` - JavaScript does not provide a native wait() function; delays are managed via timers or promises. - `sleep()` - The sleep() function is common in Python or C but is not a native JavaScript keyword. **Hint:** One-time delay. ### 27. How do you check if an object has a specific property? - `object.exists(key)` - There is no native .exists() method for checking property presence on standard objects. - **`key in object`** ✅ - The "in" operator returns true if the specified property exists in the object or its prototype chain. - `object.contains(key)` - The .contains() method is used for DOM node hierarchy checks, not for object property keys. - `object.has(key)` - While .has() is used for Map and Set collections, standard objects do not use this method. **Hint:** Checks the keys. ### 28. What is a "Closure"? - Closing browser window - Closing a window is a browser environment event and not a functional programming concept. - **Persistent scope access** ✅ - A closure is a function that retains access to its lexical scope even after that scope has closed. - Ending a loop execution - Loop termination is handled by conditions or the "break" keyword, not by closures. - A private API endpoint - Endpoints are networking concepts; a closure is an internal variable-scoping mechanism. **Hint:** A function remembering its home. ### 29. Which keyword is used to inherit from another class? - `inherits` - While accurate as a description, "inherits" is not a reserved keyword in JavaScript class syntax. - **`extends`** ✅ - The "extends" keyword is used in class declarations to create a class that is a child of another. - `implements` - The "implements" keyword is used in TypeScript and Java but is not part of the standard JS class model. - `super` - The "super" keyword is used to call functions on an object's parent, not to declare the inheritance itself. **Hint:** It "extends" the functionality. ### 30. What is the default return value of a function that doesn't return anything? - `null` - The "null" value must be returned explicitly and is not the default result for empty functions. - **`undefined`** ✅ - JavaScript automatically returns "undefined" if no specific return statement is executed. - `false` - Functions do not default to boolean values unless they are specifically designed as predicates. - `0` - The numeric value 0 is never the default return for a function without a return statement. **Hint:** It is not defined.