JavaScript 中 this的指向
this 一方面便利了讓大家在JS開發當, 但是另一方面讓開發者頭痛的是不清楚this 指代什麽.
指向全局Window:
<script> console.log(this); </script>
Function attached to global project. So "this" in the project will point to global project.
<script> function calculateAge(year) { console.log(2018 - year); console.log(this); } </script>
在object的function中的innerFunction
一些開發者認為this 應該是指向object John, 因為還在John這個 object 的scope chain中.
JS Rules:
When a regular function code called, then the default object is the window object. This is how it happens in the browser.
InnerFunction is not a method, because the method is called calculateAge. Method of the John object.
InnerFunction although is written insdie of a method, it is still a regular function.
在網頁中, 當一個普通的function 代碼被call時, default object 是 window object.
InnerFunction 這樣嵌套在method(John object裏面的calculate function), 這樣寫在method裏面的function, 是一個普通的function 而不是method, 所以this 指向全局window.
<script> varjohn = { name: ‘John‘, yearOfBirth: 1990, calculateAge: function () { console.log(this); console.log(john.yearOfBirth); function innerFunction() { console.log(this); } innerFunction(); } } john.calculateAge(); </script>
指向scope chain(作用域):
calculateAge 的function的"this"會指向當前scope chain作用域(john object)
this object refers to the object than called the method.
<script> var john = { name: ‘John‘, yearOfBirth: 1990, calculateAge: function() { console.log (this); } } john.calculateAge(); </script>
JavaScript 中 this的指向