this

this

每當被呼叫新的執行環境,就會被創造。

對類別語言而言 this 是什麼?

  • this
  • super
public class CashCard extends Card {

    private String number;
    private int balance;
    private int bonus;

    public CashCard(String number, int balance, int bonus) {
        this.number = number;
        this.balance = balance;
        this.bonus = bonus;
    }

  public getBalance() {
        return this.balance;
    }

    public static void main(String[] args) {
        CashCard cashcard = new CashCard();
    }
}

當不清楚 this 指向哪裡,你將會遇到什麼問題?

  • 無意間宣告了全域變數

範例

範例一

// function statement
function a() {
  console.log(this);
  this.myValue = 'hello';
}

// function expressions
var b = function() {
  console.log(this);
}

a();
b();
console.log(myValue);

範例二

var myObject = {
  name: 'alincode',
  log: function() {
    this.name = 'daisy';
    console.log(this);
  }
}

// 物件方法
myObject.log();

範例三

物件方法裡面宣告一個函式

var myObject = {
  name: 'alincode',
  log: function() {
    var setName = function(newname) {
      this.name = newname;    // 這裡的 this 代表的是?
      console.log(this);
    }
    setName('daisy');
  }
}

myObject.log();

範例四

解決範例三的 pattern

var myObject = {
  name: 'alincode',
  log: function() {
    var self = this;
    var setName = function(newname) {
      self.name = newname;
    }
    setName('daisy');
    console.log(self);
  }
}

myObject.log();

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