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Intermediate JavaScript

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These are slides I use in my Professional Intermediate JavaScript classes.

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Intermediate JavaScript

  1. 1. JavaScript 200 Intermediate JavaScript by Milan Adamovsky http://milan.adamovsky.com ◆ http://www.hardcorejs.com
  2. 2. Prerequisites • Fundamentals of language • Familiarity with syntax • Distinction between DOM and JS • Awareness of prototype and scope • Basic understanding of this
  3. 3. Goals • Progressive complexity • Eye on performance • Familiarity with subtleties • Explanation of some of the internals • Awareness of new ways of doing things
  4. 4. Roadmap • How to better control our code’s flow • How the code handles information • How information is resolved • How to practically apply advanced features • Introduction to more advanced topics
  5. 5. Data Structures
  6. 6. Refresher Quiz • What is an identifier? • What are valid characters in identifiers? • What type system does the language use? • What are the primitives? • What primitives are mutable?
  7. 7. Immutability • Most primitives are immutable • Type undefined is erroneously mutable • Objects are mutable • Literals are immutable • Some array methods mutate, some don’t
  8. 8. Hoisting • Identifier is known to scope before value • Function names hoist after variables • Value of variables never hoist • Declaration of functions always hoist • Reflect hoisting in coding style
  9. 9. Example function myFunction() { if (!x) console.log('undefined') else console.log('x : ', x); var x = ""; function x() { console.log('inside x function'); } } myFunction(); x : function x() { console.log('inside x function'); }
  10. 10. Performance • Primitives are best • Arrays are second best • Objects are worst • Keep nesting to a minimum • Indexes better alternatives to nesting
  11. 11. Example var family = { father : { first : 'joe', last : 'doe' }, mother : { first : 'jane', last : 'doe' } }; var father = { first : 'joe', last : 'doe' }, mother = { first : 'jane', last : 'doe' };
  12. 12. Benchmark • http://jsperf.com/registry-design • Strategic approach to data structures • Different approach, different performance • Shows nested objects are worst • Suggests multiple objects best
  13. 13. Reminders • Use typeof to determine type of variable • Use instanceOf to determine class type • Types via typeof are returned as strings • Type of null is ‘object’ • Type of NaN is ‘number’
  14. 14. Conditionals
  15. 15. Refresher Quiz • What is logic flow? • What different ways can we control it? • What is a comparison operator? • What is the triple equal operator? • What does every condition resolve for?
  16. 16. Logic Flow • if, if...else, if...else if...else • Ternary operator (?:) • Logical operators (||, &&, !) • Switch statement • Loops
  17. 17. Effective Conditions • Avoid unnecessary checks • Order conditions from most to least likely • Nest ifs to avoid redundant checks • Optional braces for single statements • Avoid function calls in conditions
  18. 18. call to multiply Example function multiply() { console.log('call to multiply'); return 100 * 100; } if (multiply() < 100) console.log('lt 100'); else if (multiply() < 100 && multiply() === 1000) console.log('lt 100 and equal to 1000'); else if (multiply() > 100 && multiply() < 100) console.log('gt 100 and lt 100'); else if (multiply() > 100 || multiply() < 100) console.log('gt 100 or lt 100'); else console.log('how did we get here?'); call to multiply call to multiply call to multiply call to multiply call to multiply gt 100 or lt 100
  19. 19. call to multiply Example function multiply() { console.log('call to multiply'); return 100 * 100; } var result = multiply(); if (result < 100) if (result === 1000) console.log('lt 100 and equal to 1000'); else console.log('lt 100'); else if (result > 100) console.log('gt 100'); else console.log('how did we get here?'); call to multiply gt 100
  20. 20. Creative Uses • Use logical operators for defaults values • Use ternary operator for assignments • Use ifs for logic flow • Nest ternary operators • Replace conditions with maps
  21. 21. Example function namePerson(firstName, lastName) { firstName = firstName || "Unknown", lastName = lastName || "Doe"; console.log("Person's name is: ", firstName, lastName); } namePerson(); namePerson("Joe"); namePerson("", "Doeson"); namePerson("Bob", "Doeson"); Person's name is: Person's name is: Person's name is: Person's name is: Unknown Doe Joe Doe Unknown Doeson Bob Doeson
  22. 22. Example var x = (firstCondition) ? (trueFirstCondition) ? firstConditionTrueValue : firstConditionFalseValue : falseFirstConditionValue; var y; if (firstCondition) if (trueFirstCondition) y = firstConditionTrueValue; else y = firstConditionFalseValue; else y = falseFirstConditionValue;
  23. 23. Objects
  24. 24. Object Literals • Lightweight • No prototype of its own • No private variables • Non-inheritable • Faster than an instantiated object
  25. 25. Example var x = { first : "one", second : "two" }; x.prototype.test = "Test!"; TypeError: Cannot set property 'test' of undefined
  26. 26. Example Object.prototype.third = "three"; var x = { first : "one", second : "two" }, y = {}; console.log(x); console.log(y); Object {first: "one", second: "two", third: "three"} Object {third: "three"}
  27. 27. Example Object.prototype.third = "three"; var x = { first : "one", second : "two" }, y = {}; console.log(delete x.third); console.log(x); console.log(delete Object.prototype.third); console.log(x); true Object {first: "one", second: "two", third: "three"} true Object {first: "one", second: "two"}
  28. 28. Instantiated Objects • Use of new operator • Comes with its own prototype • Has a constructor • Ideal for Object Oriented Programming • Permits proper encapsulation
  29. 29. Example function MyClass() { var myName = "Joe"; } MyClass.prototype.test = "Test"; var x = new MyClass(), y = {}; console.log(x); console.log(y); MyClass {test: "Test"} Object {}
  30. 30. Exercise function MyClass() { var myName = "Joe"; } Object.prototype.test = "Test 1" MyClass.prototype.test = "Test 2"; var x = new MyClass(); x.test = "Test 3"; console.log(x); MyClass {test: "Test 3", test: "Test 2", test: "Test 1"}
  31. 31. Careful • Ensure parent object always exists • Careful not to cause looping references • Accidental array to object coercions • Resist extending Object prototype • IE errors on trailing commas in literals
  32. 32. Example var x = { y : null }; x.y = x; Object {y: Object}
  33. 33. What can be done • Dynamic keys introduced via [ ] • Dot notation is optional • Use of delete to remove keys • Use of in to determine if key exists • Enumerate keys using in
  34. 34. Example var x = { "name" : "Joe" }; console.log(x.name); console.log(x['name']); var newKey = "arbitrary"; x[newKey] = "value"; x; Joe Joe Object {name: "Joe", arbitrary: "value"}
  35. 35. Look ahead • ES5 offers Object.create() • Upcoming property descriptors • Getters and setters • Proxy API in ES6 • ES5 gives access to keys via Object.keys()
  36. 36. Scope
  37. 37. Refresher • Global and functional • No block scope • The let keyword is not part of ES5 • Skipping var globalizes identifier • Scope chain
  38. 38. Scope Types • Lexical scope can be resolved lexically • Dynamic scope doesn’t exist • Lexical and global not mutually exclusive • Name binding applies with call() and apply() • Dynamics present with with() and eval()
  39. 39. Why to know • Understand how variables are resolved • Optimize code by reducing lookups • Clarify understanding of closures • Identify potential memory leaks • Detect potential name resolution conflicts
  40. 40. Exercise function A(a) { function B(b) { function C(c) { console.log(a); } C(); } B(); } var a = "Joe"; A(a); Joe
  41. 41. Functions
  42. 42. Review • Declarations • Expressions • Constructor • Anonymous functions • Type of function
  43. 43. Nesting • Limit nesting to one level if possible • Each level introduces a link in scope chain • Deteriorates performance • Careful of inadvertent nestings • Avoid wherever possible
  44. 44. Example function A(a) { B(a); } function B(a) { C(a); } function C(a) { console.log(a); } A("Joe"); output if any
  45. 45. Declarations • Best performance • Always hoist name and definition • Must be a source-element • Converts to expression for non-sourced • Using function statement
  46. 46. Example A(); function A() { console.log('A'); B(); if (!true) { function B() { console.log('B 2'); } } } function B() { console.log('B 1'); } A B2
  47. 47. Expressions • Usually anonymous • Variable is a reference to function • Named expressions useful for tracing • Name accessible within function only • Using function operator
  48. 48. Example var referenceAx = function Ax() { console.log(Ax); }; referenceAx(); Ax(); function Ax() { console.log(Ax); } ReferenceError: Ax is not defined
  49. 49. Arguments • Always passed by value • There is no by reference • Always optional • Always accessible via arguments • Array-like object but not an array
  50. 50. Example function prove(objectliteral) { objectliteral.key = true; objectliteral = undefined; } var objectliteral = {key: false}; prove(objectliteral); objectliteral.key; true
  51. 51. Example function prove(objectliteral) { var myaddress = objectliteral; // let's copy the value of the argument that supposedly // contains the address. objectliteral.key = true; objectliteral = null; myaddress.key = 1; // // // // now we should be modifying the key of the objectliteral's address which the objectliteral variable no longer has since we wiped it. Since we now have the address, we can reference the key of that object. } var objectliteral = {key: false}; prove(objectliteral); objectliteral.key; 1
  52. 52. Memoization • Reduces need for redundant calculations • Cache derived values where possible • Maintain easy-to-access registries • Use getters and setters for control • Weigh use of closures versus conditions
  53. 53. Example function resolveNumber() { var result = 100 * 100; resolveNumber = function () { return result; }; return resolveNumber(result); } console.log(resolveNumber); console.log(resolveNumber()); console.log(resolveNumber); function resolveNumber() { var result = 100 * 100; resolveNumber = function () { return result; }; return resolveNumber(result); } 10000 function () { return result; }
  54. 54. Prototype
  55. 55. Concept • Defines an object • Inheritable to inherited objects • Propagates to all new and existing objects • Has a constructor property • Good way to extend objects
  56. 56. Prototype chain • Resolve availability of method or property • Similar to scope chain • Each link traversal incurs cost • Reduce inheritance for better performance • Final link is null
  57. 57. Bad practice • Do not extend native object prototypes • Poor use of inheritance • Looping without use of hasOwnProperty • Carelessly extending an object • Don’t forget to reset constructor
  58. 58. Closures
  59. 59. Concept • Shared variables between multiple scopes • Extends variable life beyond a scope’s end • Inner scope can access outer scope • Outer scope cannot access inner scope • Locks up memory until freed
  60. 60. Example function A(x) { var b = 1; return function B() { debugger; return x; }; } var c1 = A(1); c1; function B() { debugger; return x; }
  61. 61. Expressions
  62. 62. Overview • Executed left-to-right • Comma-separate multiple expressions • Returns value of last expression • Expressions always return a value • Two types of expressions
  63. 63. Regex
  64. 64. When to use • Match patterns in text • Can be used with split() • Also available with replace() • Weigh performance versus complexity • Time is of the essence
  65. 65. Advanced features • Flags • Captures • Lookahead • Negated lookahead • Negated character set
  66. 66. Example var pattern = ".{1}s*[a-z]+"; var re = new RegExp("[0-9]" + pattern + "$", "gi"); var regex = /d{2}s*[A-Z]/gi; console.log("33v 45d".match(regex)); console.log("4@jK".match(re)); console.log(/(Joe +)(?=Doe)/.test("Joe Doe")); console.log(/(Joe) +(?=Doe)/.test("Joe Bob")); ["33v", "45d"] ["4@jK"] true false
  67. 67. Common use cases • Determine valid phone number • Determine valid e-mail address • Extract specific values from a string • Parse CSV data • Check for existence of content
  68. 68. Patterns
  69. 69. Good to know • Understand how DOM and JS interact • Memory leaks between DOM and JS • DOM transactions are costly • Patterns in JS can help with DOM • Patterns for DOM better use jQuery
  70. 70. Observer pattern • Used to decouple cause and effect • Trigger event unaware of effect • Event handlers respond to event • Similar to publish / subscribe pattern • Many frameworks use it already
  71. 71. MVC pattern • Model, View, Controller • Separation of concerns • Comes in different flavors • Ideal candidate to mix with Observer • Promotes use of templates
  72. 72. Effective Coding
  73. 73. Coding Style • Standardization improves quality • Uses Java’s naming convention de facto • Camel casing of identifiers • Uppercase first letter to suggest a Class • Alphabetize function definitions
  74. 74. Resources
  75. 75. Garbage collection • Uses a mark and sweep strategy • Assign a null value to help clean up • Use of delete to help clean up • Consistency helps with performance • Proprietary GarbageCollect() not so good
  76. 76. Debugging
  77. 77. Use of features • Breakpoints • Call stack • The debugger statement • Acceptable uses of alert() • Acceptable uses of console API
  78. 78. Console API • Bad idea to override console • Use of console.log() to navigate objects • Use of different console methods • Using console.warn() shows up in-code • Easy to shim
  79. 79. Connect • Thank you for your time • Connect with me on LinkedIn • Join the Hardcore JavaScript community • Read my blog • Contact me via e-mail

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