更改对象中的值

Question

我是javascript的新手。 我想用这些方法改变方形的长度。但它不起作用。我找不到，为什么。 Plz帮助，而不仅仅是投票。

// 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

功能似乎很好。您执行的console.logs是旧值，而不是访问对象上的属性。将sideLength设置为10后，您不会为calcPerimeter请求新的sideLength }。您需要访问sideLength对象上的square，例如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);

就像@Felix Kling提到的那样，你应该改变

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

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

根据this实际上的内容，将函数分配给this上的属性可能会产生不同的影响。在您的上下文中可能代表window对象。这就是为什么以下工作原理。

```

square.set_sideLength = set_sideLength;

```

通常，向窗口对象添加属性被认为是不好的做法。因此，您最好使用其他方法，并将整个事物包装成IIFE。

喜欢这样

(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

您还没有第二次调用calcPerimeter函数，sideLength应该调用square.sideLength。

＆＃13;

＆＃13;

// 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);

＆＃13;

＆＃13;

＆＃13;

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);

Console: