Javascript

The JavaScript Programming Language

Overview History Language Advanced Features Platforms Standards Style

The World's Most Misunderstood Programming Language

Sources of Misunderstanding The Name Mispositioning Design Errors Bad Implementations The Browser Bad Books Substandard Standard JavaScript is a Functional Language

History 1992 Oak, Gosling at Sun & FirstPerson 1995 HotJava LiveScript, Eich at Netscape 1996 JScript at Microsoft 1998 ECMAScript

Not a Web Toy It is a real language Small, but sophisticated It is not a subset of Java

Key Ideas Load and go delivery Loose typing Objects as general containers Prototypal inheritance Lambda Linkage though global variables

Values Numbers Strings Booleans Objects null undefined

Numbers Only one number type No integers 64-bit floating point IEEE-754 (aka “ Double ”) Does not map well to common understanding of arithmetic: 0.1 + 0.2 = 0.30000000000000004

NaN Special number: Not a Number Result of undefined or erroneous operations Toxic: any arithmetic operation with NaN as an input will have NaN as a result NaN is not equal to anything, including NaN

Number function Number( value ) Converts the value into a number. It produces NaN if it has a problem. Similar to + prefix operator.

parseInt function parseInt( value , 10) Converts the value into a number. It stops at the first non-digit character. The radix ( 10 ) should be required. parseInt("08") === 0 parseInt("08", 10) === 8

Math Math object is modeled on Java's Math class. It contains abs absolute value floor integer log logarithm max maximum pow raise to a power random random number round nearest integer sin sine sqrt square root

Strings Sequence of 0 or more 16-bit characters UCS-2, not quite UTF-16 No awareness of surrogate pairs No separate character type Characters are represented as strings with a length of 1 Strings are immutable Similar strings are equal ( == ) String literals can use single or double quotes

String length string .length The length property determines the number of 16-bit characters in a string.

String function String( value ) Converts value to a string

String Methods charAt concat indexOf lastIndexOf match replace search slice split substring toLowerCase toUpperCase

Boolean function Boolean( value ) returns true if value is truthy returns false if value is falsy Similar to !! prefix operator

null A value that isn't anything

undefined A value that isn't even that The default value for variables and parameters The value of missing members in objects

Falsy values false null undefined "" ( empty string) 0 NaN All other values (including all objects) are truthy. "0" "false"

Dynamic Objects Unification of Object and Hashtable new Object() produces an empty container of name/value pairs A name can be any string, a value can be any value except undefined members can be accessed with dot notation or subscript notation No hash nature is visible (no hash codes or rehash methods)

Loosely Typed Any of these types can be stored in an variable, or passed as a parameter to any function The language is not "untyped"

C JavaScript is syntactically a C family language It differs from C mainly in its type system, which allows functions to be values

Identifiers Starts with a letter or _ or $ Followed by zero or more letters, digits, _ or $ By convention, all variables, parameters, members, and function names start with lower case Except for constructors which start with upper case Initial _ should be reserved for implementations $ should be reserved for machines.

Reserved Words abstract boolean break byte case catch char class const continue debugger default delete do double else enum export extends false final finally float for function goto if implements import in instanceof int interface long native new null package private protected public return short static super switch synchronized this throw throws transient true try typeof var volatile void while with

Comments // slashslash line comment /* slashstar block comment */

Operators Arithmetic + - * / % Comparison == != < > <= >= Logical && || ! Bitwise & | ^ >> >>> << Ternary ?:

+ Addition and concatenation If both operands are numbers, then add them else convert them both to strings concatenate them '$' + 3 + 4 = '$34'

+ Unary operator can convert strings to numbers +"42" = 42 Also Number("42") = 42 Also parseInt("42", 10) = 42 +"3" + (+"4") = 7

/ Division of two integers can produce a non-integer result 10 / 3 = 3.3333333333333335

== != Equal and not equal These operators can do type coercion It is better to use === and !== , which do not do type coercion.

&& The guard operator, aka logical and If first operand is truthy then result is second operand else result is first operand It can be used to avoid null references if (a) { return a.member; } else { return a; } can be written as return a && a.member;

|| The default operator, aka logical or If first operand is truthy then result is first operand else result is second operand It can be used to fill in default values. var last = input || nr_items; (If input is truthy, then last is input, otherwise set last to nr_items .)

! Prefix logical not operator. If the operand is truthy, the result is false . Otherwise, the result is true . !! produces booleans.

Bitwise & | ^ >> >>> << The bitwise operators convert the operand to a 32-bit signed integer, and turn the result back into 64-bit floating point.

Statements expression if switch while do for break continue return try/throw

Break statement Statements can have labels. Break statements can refer to those labels. loop: for (;;) { ... if (...) { break loop ; } ... }

For statement Iterate through all of the elements of an array: for (var i = 0; i < array .length; i += 1) { // within the loop, // i is the index of the current member // array [ i ] is the current element }

For statement Iterate through all of the members of an object: for (var name in object ) { if ( object .hasOwnProperty( name )) { // within the loop, // name is the key of current member // object [ name ] is the current value } }

Switch statement Multiway branch The switch value does not need to a number. It can be a string. The case values can be expressions.

Switch statement switch ( expression ) { case ';': case ',': case '.': punctuation(); break; default: noneOfTheAbove(); }

Throw statement throw new Error( reason ); throw { name : exceptionName , message : reason };

Try statement try { ... } catch ( e ) { switch ( e .name) { case 'Error': ... break; default: throw e ; } }

Try Statement The JavaScript implementation can produce these exception names: 'Error' 'EvalError' 'RangeError' 'SyntaxError' 'TypeError' 'URIError'

With statement Intended as a short-hand Ambiguous Error-prone Don't use it with (o) { foo = null; } o.foo = null; foo = null;

Function statement function name ( parameters ) { statements ; }

Var statement Defines variables within a function. Types are not specified. Initial values are optional. var name; var nrErrors = 0; var a, b, c;

Scope In JavaScript, { blocks } do not have scope. Only functions have scope. Vars defined in a function are not visible outside of the function.

Return statement return expression ; or return; If there is no expression , then the return value is undefined . Except for constructors, whose default return value is this .

Objects Everything else is objects Objects can contain data and methods Objects can inherit from other objects.

Collections An object is an unordered collection of name/value pairs Names are strings Values are any type, including other objects Good for representing records and trees Every object is a little database

Object Literals Object literals are wrapped in { } Names can be names or strings Values can be expressions : separates names and values , separates pairs Object literals can be used anywhere a value can appear

Object Literals var myObject = {name: "Jack B. Nimble", 'goto': 'Jail', grade: 'A', level: 3}; var theName = myObject.name; var destination = myObject['goto']; 3 "level" "A" "grade" "Jail" "goto" "Jack B. Nimble" "name"

Maker Function function maker(name, where, grade, level) { var it = {}; it.name = name; it['goto'] = where; it.grade = grade; it.level = level; return it; } myObject = maker("Jack B. Nimble", 'Jail', 'A', 3);

Object Literals var myObject = {name: "Jack B. Nimble", 'goto': 'Jail', grade: 'A', format: {type: 'rect', width: 1920, height: 1080, interlace: false, framerate: 24} };

Object Literals var myObject = { name: "Jack B. Nimble", 'goto': 'Jail', grade: 'A', format: { type: 'rect', width: 1920, height: 1080, interlace: false, framerate: 24 } };

Object Literals myFunction({ type: 'rect', width: 1920, height: 1080 }); throw { name: 'error', message: 'out of bounds' };

Object Literals function SuperDiv(width, height, left, top, zIndex, position, color, visibility, html, cssClass) function SuperDiv(spec)

Object Augmentation New members can be added to any object by simple assignment There is no need to define a new class myObject.format.colorModel = 'YCgCb'; myObject[name] = value;

Linkage Objects can be created with a secret link to another object. If an attempt to access a name fails, the secret linked object will be used. The secret link is not used when storing. New members are only added to the primary object. The object( o ) function makes a new empty object with a link to object o.

Linkage var myNewObject = object(myOldObject); myNewObject myOldObject 3 "level" "A" "grade" "Jail" "goto" "Jack B. Nimble" "name"

Linkage myNewObject.name = "Tom Piperson"; myNewObject.level += 1; myNewObject.crime = 'pignapping'; "pignapping" "crime" 4 "level" "Tom Piperson" "name" 3 "level" "A" "grade" "Jail" "goto" "Jack B. Nimble" "name"

Inheritance Linkage provides simple inheritance. An object can inherit from an older object.

Prototypal Inheritance Some languages have classes, methods, constructors, and modules. JavaScript's functions do the work of all of those. Instead of Classical Inheritance, JavaScript has Prototypal Inheritance. It accomplishes the same things, but differently. It offers greater expressive power. But it's different.

Prototypal Inheritance Instead of organizing objects into rigid classes, new objects can be made that are similar to existing objects, and then customized. Object customization is a lot less work than making a class, and less overhead, too. One of the keys is the object(o) function. The other key is functions.

Object Methods All objects are linked directly or indirectly to Object.prototype All objects inherit some basic methods. None of them are very useful. hasOwnProperty( name ) Is the name a true member of this object? No copy method. No equals method.

Object Construction Make a new empty object All three of these expressions have exactly the same result: new Object() {} object(Object.prototype) {} is the preferred form.

Reference Objects can be passed as arguments to functions, and can be returned by functions Objects are passed by reference. Objects are not passed by value. The === operator compares object references, not values true only if both operands are the same object

Delete Members can be removed from an object with the delete operator delete myObject [ name ];

Arrays Array inherits from Object . Indexes are converted to strings and used as names for retrieving values. Very efficient for sparse arrays. Not very efficient in most other cases. One advantage: No need to provide a length or type when creating an array.

length Arrays, unlike objects, have a special length member. It is always 1 larger than the highest integer subscript. It allows use of the traditional for statement. for (i = 0; i < a.length; i += 1) { ... } Do not use for .. in with arrays

Array Literals An array literal uses [] It can contain any number of expressions, separated by commas myList = ['oats', 'peas', 'beans']; New items can be appended myList[myList.length] = 'barley'; The dot notation should not be used with arrays. [] is preferred to new Array() .

Array Methods concat join pop push slice sort splice

Deleting Elements delete array[number] Removes the element, but leaves a hole in the numbering. array.splice(number, 1) Removes the element and renumbers all the following elements.

Deleting Elements myArray = ['a', 'b' , 'c', 'd']; delete myArray[1]; // ['a', undefined , 'c', 'd'] myArray.splice(1, 1); // ['a', 'c', 'd']

Arrays v Objects Use objects when the names are arbitrary strings. Use arrays when the names are sequential integers. Don't get confused by the term Associative Array.

Distinguishing Arrays value.constructor === Array value instanceof Array Neither of these work when the value comes from a different frame.

Arrays and Inheritance Don’t use arrays as prototypes. The object produced this way does not have array nature. It will inherit the array's values and methods, but not its length . You can augment an individual array. Assign a method to it. This works because arrays are objects. You can augment all arrays. Assign methods to Array.prototype

Functions Functions are first-class objects Functions can be passed, returned, and stored just like any other value Functions inherit from Object and can store name/value pairs.

Function operator The function operator takes an optional name, a parameter list, and a block of statements, and returns a function object. function name ( parameters ) { statements } A function can appear anywhere that an expression can appear.

lambda What JavaScript calls function , other languages call lambda . It is a source of enormous expressive power. Unlike most power-constructs, it is secure.

Function statement The function statement is just a short-hand for a var statement with a function value. function foo() {} expands to var foo = function foo() {};

Inner functions Functions do not all have to be defined at the top level (or left edge). Functions can be defined inside of other functions.

Scope An inner function has access to the variables and parameters of functions that it is contained within. This is known as Static Scoping or Lexical Scoping.

Closure The scope that an inner function enjoys continues even after the parent functions have returned. This is called closure .

Example function fade(id) { var dom = document.getElementById(id), level = 1; function step () { var h = level.toString(16); dom.style.backgroundColor = '#FFFF' + h + h; if (level < 15) { level += 1; setTimeout(step, 100); } } setTimeout(step, 100); }

Function Objects Functions are objects, so they can contain name/value pairs. This can serve the same purpose as static members in other languages.

Method Since functions are values, functions can be stored in objects. A function in an object is called a method .

Invocation If a function is called with too many arguments, the extra arguments are ignored. If a function is called with too few arguments, the missing values will be undefined . There is no implicit type checking on the arguments.

Invocation There are four ways to call a function: Function form functionObject ( arguments ) Method form thisObject . methodName ( arguments ) thisObject [" methodName "]( arguments ) Constructor form new functionObject ( arguments ) Apply form functionObject .apply( thisObject , [ arguments ])

Method form thisObject . methodName ( arguments ) When a function is called in the method form, this is set to thisObject , the object containing the function. This allows methods to have a reference to the object of interest.

Function form functionObject ( arguments ) When a function is called in the function form, this is set to the global object. That is not very useful. It makes it harder to write helper functions within a method because the helper function does not get access to the outer this . var that = this;

Constructor form new functionObject ( arguments ) When a function is called with the new operator, a new object is created and assigned to this . If there is not an explicit return value, then this will be returned.

this this is an extra parameter. Its value depends on the calling form. this gives methods access to their objects. this is bound at invocation time. the new object constructor the object method the global object function this Invocation form

arguments When a function is invoked, in addition to its parameters, it also gets a special parameter called arguments . It contains all of the arguments from the invocation. It is an array-like object. arguments.length is the number of arguments passed.

Example function sum() { var i, n = arguments .length, total = 0; for (i = 0; i < n; i += 1) { total += arguments [i]; } return total; }

Augmenting Built-in Types Object.prototype Array.prototype Function.prototype Number.prototype String.prototype Boolean.prototype

Javascript

Javascript

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Javascript