更改對象中的值

Question

我是JavaScript新手。 我想改變使用這些方法的正方形的長度。但它不起作用。我找不到，爲什麼。 Plz幫助，而不僅僅是投票。

// to change sideLength 
 
this.set_sideLength = function(new_sideLength) { 
 
    this.sideLength = new_sideLength; 
 
}; 
 

 
var square = new Object(); 
 
square.sideLength = 6; 
 
square.set_sideLength = set_sideLength; 
 

 
// Calculate perimeter 
 
square.calcPerimeter = function() { 
 
    return this.sideLength * 4; 
 
}; 
 

 

 
var p = square.calcPerimeter(); 
 

 
// output 
 
console.log("Perimeter is: " + p + ", if side length is "+ sideLength); 
 

 
// New ouptput with new sideLength 
 
square.set_sideLength(10); 
 
console.log("Perimeter is: " + p + ", if side length is "+ sideLength);

Answer 1

的功能顯得相當精緻。您執行的console.log是舊值，並且不訪問該對象上的屬性。將sideLength設置爲10後，您不會爲該sideLength請求新的calcPerimeter。而你需要訪問sideLength的square對象像square.sideLength

// to change sideLength 
 
this.set_sideLength = function(new_sideLength) { 
 
    this.sideLength = new_sideLength; 
 
}; 
 

 
var square = new Object(); 
 
square.sideLength = 6; 
 
square.set_sideLength = set_sideLength; 
 

 
// Calculate perimeter 
 
square.calcPerimeter = function() { 
 
    return this.sideLength * 4; 
 
}; 
 

 

 
var p = square.calcPerimeter(); 
 

 
// output 
 
console.log("Perimeter is: " + p + ", if side length is "+ square.sideLength); 
 

 
// New ouptput with new sideLength 
 
square.set_sideLength(10); 
 
p = square.calcPerimeter(); 
 
console.log("Perimeter is: " + p + ", if side length is "+ square.sideLength);

像@Felix克林提到你或許應該改變

// from 
 
this.set_sideLength = function(new_sideLength) { 
 
    this.sideLength = new_sideLength; 
 
}; 
 

 
// to 
 
function set_sideLength(new_sideLength) { 
 
    this.sideLength = new_sideLength; 
 
};

分配開玩笑根據this的實際情況，可能會對this上的房產產生不同的影響。在你的上下文中可能代表window對象。這就是爲什麼以下工作。

```

square.set_sideLength = set_sideLength; 

```

一般來說它被認爲是不好的做法，將屬性添加到window對象。因此，你會更好地使用其他方法，並且整個事情都是IIFE。

像這樣

(function() { 
 
    'use strict'; 
 
// to change sideLength 
 
function set_sideLength(new_sideLength) { 
 
    this.sideLength = new_sideLength; 
 
} 
 

 
var square = new Object(); 
 
square.sideLength = 6; 
 
square.set_sideLength = set_sideLength; 
 

 
// Calculate perimeter 
 
square.calcPerimeter = function calcPerimeter() { 
 
    return this.sideLength * 4; 
 
}; 
 

 

 
var p = square.calcPerimeter(); 
 

 
// output 
 
console.log("Perimeter is: " + p + ", if side length is "+ square.sideLength); 
 

 
// New ouptput with new sideLength 
 
square.set_sideLength(10); 
 
p = square.calcPerimeter(); 
 
console.log("Perimeter is: " + p + ", if side length is "+ square.sideLength); 
 
})();

Answer 2

您還沒有叫calcPerimeter函數第二次也sideLength應該被稱爲square.sideLength。

// to change sideLength 
 
this.set_sideLength = function(new_sideLength) { 
 
    this.sideLength = new_sideLength; 
 
}; 
 

 
var square = new Object(); 
 
square.sideLength = 6; 
 
square.set_sideLength = set_sideLength; 
 

 
// Calculate perimeter 
 
square.calcPerimeter = function() { 
 
    return this.sideLength * 4; 
 
}; 
 

 

 
var p = square.calcPerimeter(); 
 

 
// output 
 
document.write("Perimeter is: " + p + ", if side length is "+ square.sideLength); 
 

 
// New ouptput with new sideLength 
 
square.set_sideLength(10); 
 
var p = square.calcPerimeter(); 
 
document.write("<br/>Perimeter is: " + p + ", if side length is "+ square.sideLength);

Answer 3

function Square(){} 
 

 
Square.prototype.setSideLength = function(sideLength) { 
 
    this.sideLength = sideLength; 
 
}; 
 

 
var square = new Square(); 
 
square.sideLength = 6; 
 

 
Square.prototype.calcPerimeter = function() { 
 
    return this.sideLength * 4; 
 
}; 
 

 
console.log("Perimeter is: " + square.calcPerimeter() + 
 
    ", if side length is "+ square.sideLength); 
 

 
square.setSideLength(10); 
 
console.log("Perimeter is: " + square.calcPerimeter() + 
 
    ", if side length is "+ square.sideLength);

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