this

Transcript

1. 1.

   this othree@JSDC

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   None
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   this • ‘the object’ it belongs in OOP

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   C++ class Box { public: Box(double l=2.0, double b=2.0, double

   h=2.0) { this->length = l; this->breadth = b; this->height = h; } double Volume() { return length * breadth * height; } int compare(Box box) { return this->Volume() > box.Volume(); } private: double length; double breadth; double height; };
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   Continued.. int main(void) { Box Box1(3.3, 1.2, 1.5); Box Box2(8.5,

   6.0, 2.0); cout << Box1.Volume() << endl; // 3.3*1.2*1.5 = 5.94 cout << Box2.Volume() << endl; // 8.5*6.0*2.0 = 102 return 0; }
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   this • Context in JavaScript • Can mean the object

   also
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   JavaScript var Box = function Box (l, b, h) {

   this.length = l; this.breadth = b; this.height = h; }; Box.prototype.Volume = function () { return this.length * this.breadth * this.height; }; Box.prototype.compare = function (box) { return this.Volume() > box.Volume(); };
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   this in Function • Context • Depends on how you

   call the function
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   3 Way to Call Function var big = function ()

   {/*...*/}; var foo = { small: function () {/*...*/} }; big(); // 1. this: window object foo.small(); // 2. this: foo var small = foo.small; small();
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    3 Way to Call Function var big = function ()

    {/*...*/}; var foo = { small: function () {/*...*/} }; big(); // 1. this: window object foo.small(); // 2. this: foo var small = foo.small; small(); // 3. this: window object
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    Borrowing Method var foo = { small: function () {

    this; } }; var bar = {}; foo.small(); // this: foo bar.borrowedSmall = foo.small; bar.borrowedSmall(); // this: bar
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    this in Global Scope • Root object

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    Root Object • `window` in browsers • Root object in

    other environment
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    to Support Both (function () { root = this; //blah....

    }());
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    Strict Mode • No more global context

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    Don’t Forget `new` function Foobar() { "use strict"; this.a =

    'foo'; this.b = 'bar'; } var foobar1 = Foobar(); // Cannot set property 'a' of undefined var foobar2 = new Foobar(); // this: new empty object
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    One More Way to Call Function

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    apply/call var foo = {}; function foobar(v1, v2) { this.bar1

    = v1; this.bar2 = v2; } foobar.call(foo, v1, v2); // this: foo foobar.apply(foo, [v1, v2]); // this: foo
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    bind var foo = {}; var otherFoo = {}; function

    setBar(v1, v2) { this.bar1 = v1; this.bar2 = v2; } var fooSetBar = setBar.bind(foo); fooSetBar(1, 2); // this: foo otherFoo.foobar = fooSetBar; otherFoo.foobar(3, 4); // this: foo
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    Protect Your Method • Bind context and function together

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    $.proxy/_.bind • Use apply to implement bind

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    Implement Bind var bind = function (func, context) { return

    function () { func.apply(context, arguments); }; };
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    Solve Event Handler • Use apply to assign context •

    JavaScript Libraries did it for you
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    this in Event Handler // W3C Dom aElement.addEventListener('click', function ()

    { this; // aElement }, false); // old IE aElement.attachEvent('onclick', function () { this; // window });
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    Callback Function function Foobar (input) { this.prefix = input; }

    Foobar.prototype.echo = function (result) { return this.prefix + result; }; fb = new Foobar(); $.get('/info', function (result) { fb.echo(result); });
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    Reduce One Stack function Foobar (input) { this.prefix = input;

    } Foobar.prototype.echo = function (result) { return this.prefix + result; }; fb = new Foobar(); $.get('/info', fb.echo); // this.prefix is 'undefined'
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    Use bind function Foobar (input) { this.prefix = input; this.echo

    = this.echo.bind(this); // Protect method } Foobar.prototype.echo = function (result) { return this.prefix + result; }; fb = new Foobar(); $.get('/info', fb.echo);
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    Cons • Performance is bad • Old browser don’t support

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    Performance http://jsperf.com/bind-vs-closure-setup/10

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    Pros • Clearer code

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    Use _.bind function Foobar (input) { this.prefix = input; this.echo

    = _.bind(this.echo, this); // function, context } function Foobar (input) { this.prefix = input; _.bindAll(this); // context } http://underscorejs.org/#bind
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    Use $.proxy function Foobar (input) { this.prefix = input; this.echo

    = $.proxy(this.echo, this); // function, context } function Foobar (input) { this.prefix = input; this.echo = $.proxy(this, 'echo'); // context, method name } http://api.jquery.com/jQuery.proxy/
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    Bind by Need fb = new Foobar(); $.get('/info', $.proxy(fb, 'echo'));

    $.get('/info', $.proxy(fb.echo, fb)); $.get('/info', $.bind(fb.echo, fb));
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    http://www.flickr.com/photos/othree/8544069132/

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    Avoid Using this

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    Closure var isIE = true; function foobar() { if (!isIE)

    { // access isIE is possible because of closure return; } // blah... };
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    that/self function Foobar(input) { var that = this; // that

    or self this.prefix = input; this.echo = function (result) { return that.prefix + result; // that is accessible because of closure }; } fb = new Foobar('res: '); $.get('/info', fb.echo);
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    CoffeeScript Fat Arrow Foobar = (input) -> @prefix = input

    @echo = (result) => @prefix + result fb = new Foobar('res: ') $.get('/info', fb.echo)
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    CoffeeScript Fat Arrow Foobar = (input) -> @prefix = input

    @echo = (result) => @prefix + result fb = new Foobar('res: ') $.get('/info', fb.echo)
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    Compile to JS var Foobar, fb; Foobar = function(input) {

    var _this = this; this.prefix = input; return this.echo = function(result) { return _this.prefix + result; }; }; fb = new Foobar('res: '); $.get('/info', fb.echo);
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    Compile to JS var Foobar, fb; Foobar = function(input) {

    var _this = this; this.prefix = input; return this.echo = function(result) { return _this.prefix + result; }; }; fb = new Foobar('res: '); $.get('/info', fb.echo);
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    Compile to JS var Foobar, fb; Foobar = function(input) {

    var _this = this; this.prefix = input; return this.echo = function(result) { return _this.prefix + result; }; }; fb = new Foobar('res: '); $.get('/info', fb.echo);
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    LiveScript use ~>

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    Pros • No more this issue, context free • Reduce

    one call stack • No call/apply, no impact on performance • Encapsulation
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    Cons • Can’t use this tip on normal constructor

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    How about AMD • Define modules can return constructor, function,

    array, primitive data • Define a singleton module on most cases • Always have data on module
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    AMD Singleton Module define('foobar', function () { return { init:

    function (prefix) { this.prefix = prefix; } echo: function (input) { return this.prefix + input; } }; });
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    Cons • Function can be borrowed • Not doing on

    right `this` you expect
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    Avoid Use this define('foobar', function () { var self =

    {}; return { init: function (prefix) { self.prefix = prefix; } echo: function (input) { return self.prefix + input; } }; });
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    Constructors? • Use bind to protect methods if necessary

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    Constructor Without this function Person(birth, gender) { var person =

    { birth: (birth || '1970/01/01'), gender: (gender || 'M') }; return { getBirth: function () { return person.birth; }, getGender: function () { return person.gender; } }; }
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    to new or not to new var p1 = Person('2013/01/02');

    p1.getBirth(); // "2013/01/02" var p2 = new Person('2000/01/02', 'F'); p2.getBirth(); // "2000/01/02" p1.getBirth(); // "2013/01/02"
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    Cons • No prototype inheritance • More memory usage for

    methods
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    Backbone Model define(function (require) { return Backbone.Model.extend( initialize: function (attrs)

    { return _.bindAll(this); }, toITEM: function () { return this.toJSON(); }, toConfig: function () { return { name: this.get('name'), package_name: this.get('package_name') }; } ); });
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    Who Use this Tip • jQuery.Deferred • jQuery.Callbacks

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    Deferred Chaining var firstDfd = $.Deferred(), secondDfd = $.Deferred(), thirdDfd

    = $.Deferred(); firstDfd.done(secondDfd.resolve); secondDfd.done(thirdDfd.resolve); firstDfd.resolve(); // All Deferred object resolved here
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    Callbacks https://github.com/jquery/jquery/blob/master/src/deferred.js // promise[ done | fail | progress ]

    = list.add promise[ tuple[1] ] = list.add; // skip... // deferred[ resolve | reject | notify ] deferred[ tuple[0] ] = function() { deferred[ tuple[0] + "With" ] (this === deferred ? promise : this, arguments); return this; }; deferred[ tuple[0] + "With" ] = list.fireWith;
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    Callbacks https://github.com/jquery/jquery/blob/master/src/deferred.js // promise[ done | fail | progress ]

    = list.add promise[ tuple[1] ] = list.add; // skip... // deferred[ resolve | reject | notify ] deferred[ tuple[0] ] = function() { deferred[ tuple[0] + "With" ] (this === deferred ? promise : this, arguments); return this; }; deferred[ tuple[0] + "With" ] = list.fireWith;
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    Actually Are promise['done'] = resolveCallbacks.add; promise['fail'] = rejectCallbacks.add; promise['progress'] =

    notifyCallbacks.add; // skip... deferred["resolveWith"] = resolveCallbacks.fireWith; deferred["rejectWith"] = rejectCallbacks.fireWith; deferred["notifyWith"] = notifyCallbacks.fireWith;
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    Summary • Understand `this` • Understand how not to use

    `this` • Use `this` carefully if necessary
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    Trade-Off • ‘Not use this’ requires more memory on methods

    definition and not easy to inheritance object • Benefit is you can write more clear, simple code
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    References

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    https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/this

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    Questions? http://www.flickr.com/photos/roman/5610736/