JavaScript - 爲什麼我不能將新屬性添加到「字符串」對象？

Question

我已經嘗試了JavaScript和注意到了這個奇怪的事情：

var s = "hello world!"; 
s.x = 5; 
console.log(s.x); //undefined 

在JavaScript中每一個類型的變量是從對象繼承。所以應該可以爲每個對象添加新的屬性。

我誤解了錯誤嗎？

Answer 1

JavaScript中的字符串不是String的實例。如果你這樣做new String('my string')那麼它會。否則，它是一個原始圖元，當您調用其上的方法時，該圖元將立即轉換爲String對象。如果你想獲得字符串的值，你需要調用toString()，如下圖所示：

var s = new String("hello world!"); 
s.x = 5; 
console.log(s.x); //5 
console.log(s); //[object Object] 
console.log(s.toString()); //hello world! 

Answer 2

String對象是對象和可擴展的，但字符串文字不是字符串對象，不能擴大。

例子：

var s = 'asdf'; 
s.x = 42; 
alert(s.x); // shows "undefined" 

s = new String('asdf'); 
s.x = 1337; 
alert(s.x); // shows "1337" 

Answer 3

Skilldrick的回答解釋了爲什麼它不工作，所以回答你的問題。

作爲一個側面說明，它是可能做到這一點：

var s = { 
    toString: function() { return "hello world!"; } 
}; 
s.x = 5; 
console.log(s.x); // 5 
console.log('result: ' + s); // "result: hello world!"; 
console.log(String(s)); // "hello world!"; 

Answer 4

你s是一個字符串，而不是字符串對象。字符串文字handled differently：

The reason you can't add properties or methods to a string literal is that when you try to access a literal's property or method, the Javascript interpreter temporarily copies the value of the string into a new object and then use that object's properties or methods. This means a String literal can only access a string's default properties or methods and those that have been added as prototypes.

Answer 5

Primitives ^{MDC docs}是不可改變的。

primitive, primitive value
A data that is not an object and does not have any methods.
JavaScript has 5 primitive datatypes: string, number, boolean, null, undefined.
With the exception of null and undefined, all primitives values have object equivalents which wrap around the primitive values, e.g. a String object wraps around a string primitive.
All primitives are immutable.

Answer 6

嘗試這樣做：

var s = "hello world!"; 
s.prototype.x = 5; 
console.log(s.x);