Question

我正在尝试创建自定义字符串方法。

我不确定如何获取字符串的值，该方法附加到该函数并传回给函数。

function testPrototyping(passedVar) {
    passedVar += " reason why this is not working";
    return passedVar;
}

String.prototype.testMethod = testPrototyping;
var myVar = "some";
var myVar2;

// 1. This works but is not what I want:
myVar2 = myVar.testMethod(myVar);

// 2. This is what I want but doesn't work:
myVar2 = myVar.testMethod();

// 3. Wondering if this should work also:
myVar2 = testMethod(myVar);

Answer 1

您需要像{}一样使用this：

function testPrototyping() {
    var text = this;

    text = text.trim();

    text + " reason why this is not working";

    return text;
}

String.prototype.testMethod = testPrototyping;

“的StackOverflow” .testMethod（）

Prototype方法中的关键字this指的是构造函数实例。您可以在此处详细了解"this"。

Answer 2

如果您想引用当前字符串，可以使用this关键字：

function testPrototyping() {
    return this + " reason why this is not working";
}

但是，您无法为this分配任何内容。所以this += "..."无效。

Answer 3

可以通过testPrototyping内的this访问对象包装器testPrototyping。 String对象包装器的实际底层字符串可通过valueOf()方法获得：

String.prototype.showStuff = function() {
  console.log(this);
  console.log(this.valueOf());
  console.log(this + ' proves that conversion is done automatically');
  console.log(typeof this, 'but it is not ok in every situation');
};
var myVar = "test";

myVar.showStuff();
/* results (Chrome 32):
String {0: "t", 1: "e", 2: "s", 3: "t", length: 4, formatUnicorn: function, truncate: function, splitOnLast: function, contains: function…}
test
test proves that conversion is done automatically
object but it is not ok in every situation 
*/

Answer 4

你不能完全按照自己的意愿去做，因为字符串在javascript中是不可变的。这意味着您无法在不创建新字符串的情况下更改字符串。

根据您真正需要实现的目标，您可以使用另一个对象来“隐藏”真正的字符串：

var MyString = function(s) {
    this.s = s;
};

MyString.prototype.toString = function() {
    return this.s;
};

MyString.prototype.testMethod = function() {
   this.s += " reason why this is working";
};

var myVar = new MyString("some");
alert(myVar); // "some"
myVar.testMethod();
alert(myVar); // "some reason why this is working"

JavaScript - 使用String原型函数做错了

4 个答案: