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JavaScript Developer

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JavaScript Developer

Study Checklist

  • Platform Administrator
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  • Marketing Cloud Engagement Administrator
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Checklist progress

0/168Learned

  • Difference between var, let, and const in terms of scope and hoisting behavior
  • Block scoping with let and const versus function scoping with var
  • How var declarations are hoisted to the top of their function scope and initialized to undefined
  • Temporal dead zone (TDZ) behavior for let and const declarations before initialization
  • Destructuring assignment for arrays and objects to extract and initialize variables
  • Rest parameters (...args) for collecting variable-length arguments into an array
  • Classic var-in-for-loop closure trap: all loop iterations share one var binding versus per-iteration let binding
  • Default parameter values in function signatures
  • Template literals (backtick strings) for multi-line strings and expression interpolation
  • String search methods: indexOf, includes, startsWith, endsWith
  • String extraction and splitting: slice, substring, split
  • String modification methods: replace, trim, padStart, padEnd, repeat
  • Number parsing: Number.parseInt and Number.parseFloat
  • Number formatting with toFixed for decimal places
  • Number validation methods: Number.isInteger, Number.isFinite, Number.isNaN
  • Math rounding methods: Math.floor, Math.ceil, Math.round
  • Math comparison and absolute value: Math.max, Math.min, Math.abs
  • Date object creation with new Date() and getter methods: getFullYear, getMonth, getDate, getTime
  • Date serialization: toISOString and toLocaleDateString for string output
  • typeof operator return values for all primitive types and objects (including typeof null === 'object')
  • Explicit type conversion using String(), Number(), Boolean(), parseInt(), and parseFloat()
  • Implicit type coercion with the + operator when mixing strings and numbers
  • Abstract equality (==) versus strict equality (===) and the coercion rules applied
  • Coercion behavior when using arithmetic operators on null, undefined, and boolean values
  • NaN comparison behavior: NaN !== NaN is true; use Number.isNaN() to detect it
  • Optional chaining operator (?.) for safely accessing nested properties that may be null or undefined
  • The six falsy values in JavaScript: false, 0, '', null, undefined, NaN
  • Truthy evaluation of objects, arrays, non-zero numbers, and non-empty strings in conditional expressions
  • Short-circuit evaluation with && and || operators using truthy/falsy logic
  • Nullish coalescing operator (??) versus logical OR (||) for default value assignment
  • Array methods that mutate the original array: push, pop, shift, unshift, splice, sort, reverse, fill
  • Array methods that return a new array: map, filter, slice, concat, flat, flatMap
  • Array iteration methods: forEach, reduce, find, findIndex, some, every, includes
  • for...of loop for iterating over arrays and other iterables versus for...in for object keys
  • Sorting arrays of objects using a comparator function with Array.prototype.sort
  • Array destructuring to extract values into named variables including skipping elements
  • Spread operator (...) for cloning arrays and combining arrays
  • Array.from() for creating arrays from array-like objects and iterables
  • JSON.parse() to convert a JSON string into a JavaScript object
  • Accessing nested properties of a parsed JSON object using dot and bracket notation
  • JSON.stringify() to serialize a JavaScript object to a JSON string, including the replacer and space parameters
  • JSON.stringify behavior with undefined, functions, and Symbol values (they are omitted)
  • Function declaration hoisting versus function expression (not hoisted) behavior
  • Arrow functions versus regular functions: syntax differences and behavior of the this keyword
  • call(), apply(), and bind() methods for explicitly setting the this context of a function
  • Closures: inner functions retaining access to outer function's variable scope after the outer function returns
  • Higher-order functions: functions that accept or return other functions
  • Pure functions versus functions with side effects, and the importance of referential transparency
  • Immediately Invoked Function Expressions (IIFE) syntax and use case for creating private scope
  • Generator functions (function*) with yield: creating iterators for lazy value generation
  • Object literal syntax: shorthand property names, computed property names, and method shorthand
  • Prototype chain and how property lookup traverses the chain
  • Object.create() for creating an object with a specified prototype (prototypal inheritance without classes)
  • instanceof operator: checking whether an object's prototype chain includes the prototype of a given constructor
  • hasOwnProperty() and Object.hasOwn() for distinguishing own properties from inherited prototype properties
  • Object.keys(), Object.values(), Object.entries() for iterating over an object's own enumerable properties
  • Object spread operator (...) for cloning and merging objects (shallow copy)
  • Object.assign() for shallow copying or merging objects
  • Map collection: key-value pairs, its methods, and when to use over plain objects
  • Set collection: unique values only, its methods, and when to use over plain arrays
  • Object.freeze() and its effect on object mutability (prevents all changes)
  • Object destructuring with renaming and default values
  • ES6 class declaration syntax: constructor, instance methods, and creating instances with new
  • Classes are syntactic sugar over prototype-based inheritance: the prototype chain still underlies class instances
  • Class inheritance with extends and super() for calling the parent constructor and parent methods
  • Public class fields: declaring instance properties directly in the class body without the constructor
  • Getters and setters in classes using get and set keywords
  • Static methods and static properties on classes: when and how they differ from instance methods
  • Private class fields (#fieldName) and their access restriction within the class body
  • Module scope: variables declared in a module are not global and are private to that module by default
  • ES module syntax: named exports versus default exports
  • Importing a default export and the difference from named import syntax
  • Importing named exports with destructuring syntax and renaming with 'as'
  • CommonJS module syntax (require/module.exports) versus ES module syntax (import/export)
  • Dynamic import() for lazy loading modules and its Promise-based return
  • Decorator syntax (@decoratorName) placement on classes and class members
  • How a method decorator receives target, propertyKey, and descriptor arguments
  • How a class decorator receives the constructor as its argument and can modify or wrap it
  • Execution context and the call stack: how function calls push and pop execution contexts
  • Lexical scoping: how inner functions access variables from enclosing scopes at the time the function is defined
  • The value of this in different contexts: global, method call, constructor call, and arrow functions
  • Variable shadowing: inner scope declaration hiding an outer scope variable of the same name
  • Common event types: click, input, change, submit, keydown, keyup, focus, blur, load, DOMContentLoaded
  • addEventListener syntax: event type, listener callback, and optional useCapture/options parameter
  • Event bubbling versus event capturing phases and the order events propagate through the DOM
  • event.target versus event.currentTarget: the element that triggered the event versus the element the listener is attached to
  • event.stopPropagation() to halt event bubbling
  • event.preventDefault() to cancel the default browser action
  • Event delegation: attaching a single listener on a parent element to handle events from child elements using event.target
  • removeEventListener: correct usage requires a reference to the same function instance used in addEventListener
  • Custom events using new CustomEvent() with a detail payload and dispatching with dispatchEvent()
  • Single-element DOM selection: getElementById and querySelector
  • Collection DOM selection: querySelectorAll, getElementsByClassName, getElementsByTagName (live HTMLCollection vs static NodeList)
  • Traversing the DOM: parentNode, children, firstElementChild, lastElementChild, nextElementSibling
  • Creating DOM elements with createElement
  • Inserting DOM nodes: appendChild, append, insertBefore, prepend
  • Removing DOM nodes: removeChild, element.remove()
  • Modifying element content: textContent versus innerHTML (and security implications of innerHTML with user input)
  • Modifying element attributes: getAttribute, setAttribute, removeAttribute, dataset property
  • classList API: add, remove, toggle, contains for CSS class manipulation
  • Inline style manipulation via element.style and reading computed styles with getComputedStyle()
  • Using the console panel: console.log, console.error, console.warn, console.table, console.group
  • Setting breakpoints in the Sources panel and stepping through code (Step Over, Step Into, Step Out)
  • Using the Watch and Scope panels to inspect variable values at a breakpoint
  • Inspecting network requests in the Network panel: request/response headers, status codes, payload
  • Fetch API: making GET and POST requests, handling the Response object, and parsing JSON
  • Fetch API request options: method, headers, body, mode, and credentials
  • setTimeout and setInterval: scheduling code execution and clearing timers with clearTimeout/clearInterval
  • Web Storage API: setItem, getItem, removeItem, and clear methods
  • Difference between localStorage (persists until cleared) and sessionStorage (cleared when tab closes)
  • window.location properties for reading URL parts: href, pathname, search, hash
  • window.location methods for navigation: assign(), replace(), reload()
  • URL and URLSearchParams APIs for constructing and parsing URLs and query parameters
  • History API: pushState, replaceState, popstate event for single-page app navigation
  • requestAnimationFrame for scheduling visual updates in sync with the browser repaint cycle
  • try/catch/finally block structure: what executes in each block and when finally runs
  • The Error object properties: message, name, and stack
  • Throwing custom errors: throw new Error('message') and throw new TypeError, RangeError, etc.
  • Catching errors in async/await code using try/catch versus .catch() on a Promise chain
  • Re-throwing errors in a catch block to propagate to a higher handler
  • Creating custom Error subclasses using class MyError extends Error with a constructor that sets name
  • finally block behavior when a return or throw statement appears inside try or catch blocks
  • The debugger statement as a programmatic breakpoint in source code
  • Conditional breakpoints: pausing execution only when a specified condition is true
  • console.time() and console.timeEnd() for measuring elapsed time between two points in code
  • console.assert() for logging an error message only when a condition is false

Given a scenario, apply asynchronous programming concepts.

0/8

  • Promise states: pending, fulfilled, and rejected, and how a Promise transitions between states
  • Promise chaining with .then(), .catch(), and .finally(): return values passed to the next .then()
  • async/await syntax: marking a function async so it returns a Promise and using await to pause execution
  • The return value of an async function and how it is automatically wrapped in a resolved Promise
  • Sequential versus parallel async operations: using await in a loop versus Promise.all with map
  • Promise.all(): runs promises in parallel and resolves when all resolve, or rejects on first rejection
  • Promise.allSettled(): resolves with an array of outcome objects regardless of individual promise results
  • Promise.race(): resolves or rejects as soon as the first promise in the iterable settles

Given a scenario, use event loop and event monitor or determine loop outcomes.

0/4

The JavaScript event loop: call stack, Web APIs, callback queue (macro-task queue), and microtask queue

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Microtasks (Promise callbacks) execute before the next macro-task (setTimeout callback)

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Output ordering when mixing synchronous code, setTimeout(fn, 0), and resolved Promise .then() callbacks

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How unhandled Promise rejections are detected and surfaced in Node.js vs the browser

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  • Node.js event-driven, non-blocking I/O model and when it is appropriate versus thread-based approaches
  • The process object: process.argv, process.env, process.exit(), process.stdout, and process.cwd()
  • Reading and writing files with the fs module: fs.readFile, fs.writeFile, fs.appendFile (callback and promise variants)
  • Creating an HTTP server with the http module using http.createServer and server.listen
  • node command: running a script with node filename.js and passing arguments via process.argv
  • npx: running a package binary without installing it globally, and how it differs from npm run
  • Environment variables in Node.js via process.env and setting them at the command line
  • Core Node.js built-in modules: path, os, fs, http, https, events, crypto, util, url
  • path.join() and path.resolve() for constructing cross-platform paths
  • Express.js routing: defining routes with HTTP methods and path patterns
  • Express.js request and response objects: properties and methods for handling HTTP
  • Express.js middleware: order of execution, next(), and modifying request/response
  • package.json structure: name, version, scripts, dependencies, devDependencies, and engines fields
  • npm install versus npm install --save-dev: difference between runtime and development dependencies
  • npm scripts: defining and running custom commands in the scripts section of package.json
  • Semantic versioning (semver): major, minor, and patch version increments and the ^ and ~ range specifiers
  • npm versus yarn: key command differences and use cases for each
  • package-lock.json and yarn.lock: purpose of lock files in reproducible builds
  • Unit tests versus integration tests versus end-to-end tests: scope, speed, and isolation tradeoffs
  • Test structure with describe, it/test, beforeEach, afterEach, beforeAll, afterAll blocks
  • Common Jest/Jasmine matchers: toBe, toEqual, toBeTruthy, toBeFalsy, toThrow, toHaveBeenCalledWith
  • toBe versus toEqual: reference equality versus deep structural equality for objects and arrays
  • Identifying a test that does not assert the correct output and rewriting the assertion
  • Testing asynchronous code: using async/await or returning a Promise in a test case
  • Mocking and stubbing external dependencies (e.g., fetch, database calls) to isolate the unit under test
  • jest.fn() for creating mock functions versus jest.spyOn() for spying on existing methods
  • Spy functions: tracking calls and arguments to a function without altering its behavior
  • jest.mock() for auto-mocking an entire module so all its exports become jest.fn() stubs
  • toHaveBeenCalled, toHaveBeenCalledTimes, and toHaveBeenCalledWith matchers for verifying mock invocations
  • Test coverage concepts: identifying code paths (branches, lines) not exercised by a test suite

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Output ordering when mixing synchronous code, setTimeout(fn, 0), and resolved Promise .then() callbacks

Explainer

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Practice Question

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Next conceptHow unhandled Promise rejections are detected and surfaced in Node.js vs the browser

Checklist progress

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0 of 168 concepts learned

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Prepare for the Exam

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Explainer

JavaScript is single-threaded and processes work via an event loop. Synchronous code runs to completion first; then microtasks (Promise callbacks) run; then macrotasks (setTimeout, setInterval, I/O). Understanding this ordering predicts output when synchronous code, setTimeout(fn, 0), and resolved Promise .then() callbacks are mixed.

Core information
  • The call stack executes synchronous code: function calls push frames onto the stack; returns pop them off. The event loop waits for the stack to empty before processing queued work.
More details and nuances
  • Order of a typical mixed snippet: synchronous code → resolved Promise .then() callbacks (microtasks) → setTimeout callbacks (macrotasks). Example: console.log('A'); Promise.resolve().then(() => console.log('B')); setTimeout(() => console.log('C'), 0); outputs A → B → C.