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JavaScript Developer

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JavaScript Developer Study Checklist
  • Platform Administrator
  • Platform App Builder
  • Platform Foundations
  • Platform Developer
  • Platform Administrator II
  • Agentforce Sales Consultant
  • Agentforce Service Consultant
  • Platform Data Architect
  • Platform Development Lifecycle and Deployment Architect
  • Platform Identity and Access Management Architect
  • Platform Integration Architect
  • Platform Sharing and Visibility Architect
  • Heroku Architect
  • B2C Solution Architect
  • Experience Cloud Consultant
  • Agentforce Field Service and Operations Consultant
  • Agentforce Nonprofit Consultant
  • Data 360 Consultant
  • Omnistudio Consultant
  • CRM Analytics and Einstein Discovery Consultant
  • Platform User Experience Designer
  • Platform Strategy Designer
  • B2C Commerce Developer
  • JavaScript Developer
  • Omnistudio Developer
  • Platform Developer II
  • Marketing Cloud Engagement Administrator
  • Marketing Cloud Engagement Specialist
  • Marketing Cloud Engagement Consultant
  • Agentforce Sales Foundations
  • Business Analyst
  • Marketing Cloud Engagement Developer
  • Marketing Cloud Engagement Foundations
  • Agentforce Specialist
  • Agentforce Life Sciences Consultant
  • B2B Commerce Administrator AP
  • B2B Commerce Developer AP
  • Agentforce Consumer Goods AP
  • Agentforce Financial Services AP
  • Agentforce Health AP
  • Agentforce Manufacturing AP
  • MuleSoft Integration Foundations
  • MuleSoft Developer
  • MuleSoft Developer II
  • MuleSoft Platform Integration Architect
  • MuleSoft Platform Architect
  • Tableau Desktop Foundations
  • Tableau Data Analyst
  • Tableau Consultant
  • Tableau Server Administrator
  • Tableau Architect
  • Slack Administrator

Checklist progressProgress

0/233Learned

  • Legal variable identifiers and case sensitivity
  • Primitive data types and typeof results for declared values
  • undefined, null and undeclared variables
  • Block scope versus function scope for var, let and const
  • const declaration initializer and reassignment rules
  • const bindings holding mutable objects and arrays
  • Object literal property shorthand and computed property names
  • Array destructuring with skipped, default and rest elements
  • Object destructuring with renaming, defaults and nesting
  • Redeclaration errors versus shadowing in nested blocks
  • Hoisting of var versus the temporal dead zone for let and const
  • Loop counters declared with let versus var inside closures
  • Template literals for interpolation and multiline text
  • String case, trim and pad methods on immutable strings
  • Extracting characters and substrings from strings
  • String search methods and their return values
  • Replacing text with replace, replaceAll, and regular expressions
  • Arithmetic operator precedence, remainder and exponent
  • Prefix versus postfix increment and compound assignment
  • Number() versus parseInt(), parseFloat() and unary plus
  • Correcting unintended concatenation of numeric strings
  • NaN and Infinity results and their detection
  • Math object rounding, range, and random methods
  • Rounding and formatting numbers for display
  • Floating-point precision, safe integers and BigInt
  • Creating Date objects from components, strings, and timestamps
  • Date get and set methods and component ranges
  • Date arithmetic and comparison with timestamps
  • Formatting dates and handling UTC versus local time
  • String concatenation versus numeric addition with the plus operator
  • Implicit number conversion by arithmetic and unary operators
  • String conversion with String(), toString() and template literals
  • Loose equality versus strict equality comparisons
  • Relational comparisons between strings, numbers and null
  • Object and array conversion to primitives in expressions
  • Property key coercion to strings in objects and arrays
  • Primitive auto-wrapping versus explicit wrapper objects
  • Falsy values in conditional evaluation
  • Truthiness of empty arrays, empty objects and non-empty strings
  • Explicit boolean conversion with double NOT and Boolean()
  • Operand values returned by logical OR and AND
  • Short-circuit evaluation and skipped side effects
  • Logical OR defaults versus nullish coalescing for valid falsy inputs
  • Truthiness versus loose equality with true or false
  • Array creation with literals, the Array constructor and Array.from
  • Array length, index assignment and empty slots
  • Adding and removing elements at either end of an array
  • splice for inserting, removing and replacing elements
  • for...of versus for...in versus forEach over arrays
  • forEach versus map for transforming elements
  • filter predicates and the elements they keep
  • Searching arrays and testing elements against a condition
  • reduce accumulators and the initial value
  • Chaining filter, map and reduce over a list of objects
  • Converting between arrays and strings
  • Default array sort order versus comparator functions
  • Mutating versus non-mutating array methods
  • Copying and merging arrays versus sharing a reference
  • Nested arrays and flattening
  • Set and Map keyed collections for deduplication and lookup
  • JSON text versus parsed values with parse and stringify
  • Valid JSON syntax versus JavaScript object literals
  • Navigating nested values in a parsed JSON response
  • Optional chaining versus the TypeError from reading through missing objects
  • Reading JSON from a fetch response with response.json()
  • JSON.stringify output for special values, Dates and toJSON
  • Replacer, space and reviver arguments of JSON methods
  • Deep copying objects with a JSON round-trip

Given a business requirement, locate the best function implementation

0/11

  • Function parameters, arguments, and return values
  • Primitive versus object arguments passed to functions
  • Arrow function syntax and implicit returns
  • When to use arrow functions versus regular functions
  • Default parameters versus logical OR fallbacks
  • Destructured options parameters with default values
  • Rest parameters versus the arguments object
  • Spread syntax versus rest syntax around function calls
  • Higher-order functions and callback references
  • Recursive functions and base cases
  • Generator functions and yield

Given a business requirement, apply fundamentals of object implementation to solve the business requirement

0/10

  • Object literal syntax: methods, shorthand properties, and JSON contrast
  • Dot versus bracket notation and dynamic property keys
  • Checking for and deleting object properties
  • Enumerating object keys, values, and entries
  • Object references, mutation, and identity comparison
  • Copying and merging objects with spread and Object.assign
  • Constructor functions and the new operator
  • Prototype chain lookup, property shadowing, and instanceof
  • Choosing between object literals, constructor functions, and Object.create
  • Restricting object changes with freeze, seal, and non-writable properties

Given a business requirement, apply fundamentals of class implementation to solve the business requirement

0/9

  • ES6 class syntax versus prototype-based constructor functions
  • Class body member syntax and constructor rules
  • Class declaration hoisting and class expressions
  • Static methods and fields versus instance methods
  • Getters and setters in classes and object literals
  • Public class fields and instance initialization order
  • Private class fields and methods with # syntax
  • Derived class constructors and super() calls
  • Method overriding and calling parent methods via super

Given a JavaScript module, give examples of how to use the module

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Named exports and matching named imports

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Default export and default import syntax

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Renaming imports and exports with the as keyword

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Namespace imports and the module namespace object

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Re-exporting and aggregating modules through a barrel file

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Loading ES modules in the browser with script type module

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Module scope, strict mode, and one-time evaluation

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Read-only live bindings of imported values

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Dynamic import() for loading modules on demand

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CommonJS require versus ES module import in Node.js

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Given a block of code, analyze the variable scope and the execution flow

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  • Lexical scope chain resolution in nested functions
  • Variable shadowing across global, function, and block scopes
  • Global scope bindings and implicit globals in strict versus sloppy mode
  • Function declarations versus function expressions and hoisting
  • break, continue, and switch fall-through in execution flow
  • Closures retaining and updating captured variable state
  • Private state with closures and the IIFE module pattern
  • this context determined by how a function is invoked
  • call, apply, and bind for an explicit this context
  • Preserving this inside nested callbacks
  • Event handler properties versus addEventListener
  • Registering a handler: function reference versus invocation
  • Removing event listeners and one-time listeners
  • Choosing mouse, keyboard, focus and form event types
  • Pointer events for mouse, touch and pen input
  • The event object: target, currentTarget and this in handlers
  • preventDefault and default browser actions
  • Event bubbling order through ancestor elements
  • Capture phase listeners and mixed capture/bubble ordering
  • Multiple listeners on one target: invocation order and duplicates
  • stopPropagation versus stopImmediatePropagation
  • Event delegation for dynamically added child elements
  • Creating and dispatching events: CustomEvent and synchronous dispatchEvent
  • Debouncing and throttling high-frequency event handlers
  • DOM tree nodes and traversal properties
  • DOM element selection methods and what they return
  • textContent versus innerText versus innerHTML
  • HTML attributes versus DOM element properties
  • Styling elements with classList, className and the style property
  • Creating, inserting, moving and removing DOM nodes
  • Page load events and script timing before DOM manipulation
  • Choosing the right DevTools panel for an investigation task
  • Inspecting paused state with Watch, Scope and Call Stack
  • Using the Console in a paused scope to evaluate and modify variables
  • Pause on caught versus uncaught exceptions
  • Browser host objects and Web APIs beyond core JavaScript
  • Choosing client-side storage: localStorage, sessionStorage or cookies
  • Web Storage reads and writes with string-only values
  • Timer APIs for scheduling and cancelling callbacks
  • Fetch API requests and Response handling
  • History API navigation methods
  • pushState, replaceState, and the popstate event
  • try...catch...finally forms and execution flow
  • Built-in error types raised by common runtime mistakes
  • Error name, message and cause properties
  • Throwing Error objects versus arbitrary values
  • Custom error classes extending Error
  • Selective catching with instanceof and rethrowing
  • Exception propagation through nested try blocks and the call stack
  • Return and throw statements inside finally blocks
  • Errors that escape try...catch in asynchronous code
  • Console logging methods and log levels
  • Choosing console methods for structured, counted and timed output
  • Runtime assertions with console.assert
  • Tracing the call path with console.trace and the call stack
  • The debugger statement versus DevTools line-of-code breakpoints
  • Stepping controls while paused: step over, into, out and resume
  • Conditional breakpoints for pausing on a specific value or iteration
  • Choosing a DevTools breakpoint type for a debugging scenario
  • Callback functions and the error-first callback convention
  • Callback hell versus flat promise chains
  • Promise states and settlement terminology
  • Promise constructor settlement rules for resolve, reject and throw
  • Wrapping callback-based APIs in a Promise
  • Values passed along a then() chain
  • Rejection propagation and recovery in promise chains
  • catch() versus then() rejection handlers and nested catch scope
  • finally() handler behavior in promise chains
  • Consuming fetch() responses in a promise chain
  • Promise.all versus Promise.allSettled
  • Promise.race versus Promise.any
  • Async function return values and reading them with await
  • Where await is permitted and async declaration forms
  • Error handling with try/catch around await
  • Sequential awaits versus concurrent Promise.all
  • Preserving this in timer and asynchronous callbacks
  • Call stack, task queue and event loop roles in the runtime
  • Synchronous statements versus zero-delay setTimeout callbacks
  • Multiple timer ordering and delays as minimum waits
  • setTimeout callbacks inside loops with var versus let
  • Promise microtasks versus setTimeout macrotasks
  • Promise executor timing versus then handler timing
  • Async function execution before and after await
  • Tasks and microtasks scheduled from inside other callbacks
  • await inside forEach versus for...of loops
  • Synchronous dispatchEvent versus natively queued events
  • Node.js runtime globals versus browser-only APIs
  • Non-blocking I/O in Node.js and the workloads it suits
  • CommonJS versus ES modules in a Node.js project
  • CommonJS exports versus module.exports
  • Basic HTTP server with the Node.js http module
  • Using cluster, worker_threads and child_process in a Node.js app
  • Launching a Node.js app with node versus npm start
  • Node.js REPL sessions and dot commands
  • Passing arguments and environment variables to a node process
  • Common node command-line options
  • Debugging a Node.js script from the command line
  • Matching a requirement to a core Node.js module
  • fs callback, synchronous, and promise-based APIs
  • EventEmitter custom events with the events module
  • Streams versus whole-file reads for large data
  • Outbound HTTP and HTTPS requests from a Node.js server
  • Bare http module versus the Express framework
  • Choosing a Node.js framework or library for a requirement
  • package.json manifest fields and npm init scaffolding
  • dependencies versus devDependencies versus peerDependencies
  • Local versus global package installs and npx
  • npm scripts and lifecycle hooks in package.json
  • Semantic versioning increments when releasing a package
  • Semver range operators in package.json dependencies
  • package-lock.json and npm ci versus npm install
  • npm workspaces in a multi-package project
  • npm versus Yarn and other Node.js package managers
  • Unit, integration, and end-to-end test scope
  • JavaScript testing tools and ways to run tests
  • console.assert checks as lightweight unit tests
  • BDD test suite anatomy with describe, it and expect
  • Matcher choice and assertion strength
  • Happy-path-only tests that miss branches and boundary values
  • Negative-path tests for invalid input and thrown errors
  • Async tests that finish before the promise or callback settles
  • Test isolation with setup and teardown hooks
  • Stubbing non-deterministic and external dependencies
  • Mock functions to verify callback and event-handler calls
  • Behaviour versus implementation-detail tests, false positives and negatives

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  1. JavaScript Developer Checklist
  3. Objects, Functions, and Classes

Module scope, strict mode, and one-time evaluation

Explainer

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Practice Question

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Next conceptRead-only live bindings of imported values

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  1. JavaScript Developer Checklist
  3. Objects, Functions, and Classes
Module scope, strict mode, and one-time evaluation

Explainer

Learn More

Practice Question

Keep going

Next conceptRead-only live bindings of imported values

Explainer

An ES6 module is a file of JavaScript that the runtime treats as a module, for example a file loaded with <script type="module">. Modules act differently from classic scripts in three ways that often come up in code questions. Their top-level names stay private to the file, their code always runs in strict mode, and each module runs only once, right when it is loaded.
Core information
  • Everything inside a module is scoped to that module only. A function, variable or class declared in one module is invisible to other code unless the module exports it and the other code imports it.
More details and nuances
  • A file does not need an import or export statement to be a module. The only requirement is that the entry point has type="module". If a <script> tag without that attribute contains an import declaration, a SyntaxError is thrown. The same happens inside eval and new Function.

Explainer

An ES6 module is a file of JavaScript that the runtime treats as a module, for example a file loaded with <script type="module">. Modules act differently from classic scripts in three ways that often come up in code questions. Their top-level names stay private to the file, their code always runs in strict mode, and each module runs only once, right when it is loaded.
Core information
  • Everything inside a module is scoped to that module only. A function, variable or class declared in one module is invisible to other code unless the module exports it and the other code imports it.
More details and nuances
  • A file does not need an import or export statement to be a module. The only requirement is that the entry point has type="module". If a <script> tag without that attribute contains an import declaration, a SyntaxError is thrown. The same happens inside eval and new Function.