Intermediate JavaScript

JavaScript 200
Intermediate JavaScript
by Milan Adamovsky
http://milan.adamovsky.com ◆ http://www.hardcorejs.com

Prerequisites
• Fundamentals of language
• Familiarity with syntax
• Distinction between DOM and JS
• Awareness of prototype and scope
• Basic understanding of this

Goals
• Progressive complexity
• Eye on performance
• Familiarity with subtleties
• Explanation of some of the internals
• Awareness of new ways of doing things

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

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?

Immutability
• Most primitives are immutable
• Type undefined is erroneously mutable
• Objects are mutable
• Literals are immutable
• Some array methods mutate, some don’t

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

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');
}

Performance
• Primitives are best
• Arrays are second best
• Objects are worst
• Keep nesting to a minimum
• Indexes better alternatives to nesting

Example
var family = {
father : {
first : 'joe',
last : 'doe'
},
mother : {
first : 'jane',
last : 'doe'
}
};
var father = {
first : 'joe',
last : 'doe'
},
mother = {
first : 'jane',
last : 'doe'
};

Benchmark
• http://jsperf.com/registry-design
• Strategic approach to data structures
• Different approach, different performance
• Shows nested objects are worst
• Suggests multiple objects best

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’

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?

Logic Flow
• if, if...else, if...else if...else
• Ternary operator (?:)
• Logical operators (||, &&, !)
• Switch statement
• Loops

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

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

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

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

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

Example
var x = (firstCondition)
? (trueFirstCondition)
? firstConditionTrueValue
: firstConditionFalseValue
: falseFirstConditionValue;
var y;
if (firstCondition)
if (trueFirstCondition)
y = firstConditionTrueValue;
else
y = firstConditionFalseValue;
else
y = falseFirstConditionValue;

Object Literals
• Lightweight
• No prototype of its own
• No private variables
• Non-inheritable
• Faster than an instantiated object

Example
var x = {
first : "one",
second : "two"
};
x.prototype.test = "Test!";

TypeError: Cannot set property 'test' of undefined

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"}

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"}

Instantiated Objects
• Use of new operator
• Comes with its own prototype
• Has a constructor
• Ideal for Object Oriented Programming
• Permits proper encapsulation

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 {}

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"}

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

Example
var x = {
y : null
};
x.y = x;
Object {y: Object}

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

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"}

Look ahead
• ES5 offers Object.create()
• Upcoming property descriptors
• Getters and setters
• Proxy API in ES6
• ES5 gives access to keys via Object.keys()

Refresher
• Global and functional
• No block scope
• The let keyword is not part of ES5
• Skipping var globalizes identifier
• Scope chain

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()

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

Exercise
function A(a)
{
function B(b)
{
function C(c)
{
console.log(a);
}
C();
}
B();
}
var a = "Joe";
A(a);

Joe

Review
• Declarations
• Expressions
• Constructor
• Anonymous functions
• Type of function

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

Example
function A(a)
{
B(a);
}
function B(a)
{
C(a);
}
function C(a)
{
console.log(a);
}
A("Joe");

output if any

Declarations
• Best performance
• Always hoist name and definition
• Must be a source-element
• Converts to expression for non-sourced
• Using function statement

Example
A();
function A()
{
console.log('A');
B();
if (!true)
{
function B()
{
console.log('B 2');
}
}
}
function B()
{
console.log('B 1');
}

A
B2

Expressions
• Usually anonymous
• Variable is a reference to function
• Named expressions useful for tracing
• Name accessible within function only
• Using function operator

Example
var referenceAx = function Ax()
{
console.log(Ax);
};
referenceAx();
Ax();

function Ax()
{
console.log(Ax);
}
ReferenceError: Ax is not defined

Arguments
• Always passed by value
• There is no by reference
• Always optional
• Always accessible via arguments
• Array-like object but not an array

Example
function prove(objectliteral)
{
objectliteral.key = true;
objectliteral = undefined;
}
var objectliteral = {key: false};
prove(objectliteral);
objectliteral.key;

true

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

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

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; }

Concept
• Defines an object
• Inheritable to inherited objects
• Propagates to all new and existing objects
• Has a constructor property
• Good way to extend objects

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

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

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

Example
function A(x)
{
var b = 1;
return function B()
{
debugger;
return x;
};
}
var c1 = A(1);
c1;

function B()
{
debugger;
return x;
}

Overview
• Executed left-to-right
• Comma-separate multiple expressions
• Returns value of last expression
• Expressions always return a value
• Two types of expressions

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

Advanced features
• Flags
• Captures
• Lookahead
• Negated lookahead
• Negated character set

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

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

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

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

MVC pattern
• Model, View, Controller
• Separation of concerns
• Comes in different flavors
• Ideal candidate to mix with Observer
• Promotes use of templates

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

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

Use of features
• Breakpoints
• Call stack
• The debugger statement
• Acceptable uses of alert()
• Acceptable uses of console API

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

Connect
• Thank you for your time
• Connect with me on LinkedIn
• Join the Hardcore JavaScript community
• Read my blog
• Contact me via e-mail

Intermediate JavaScript

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