String.prototype的“this”不返回字符串？

Question

What is going on here? Just when I thought I knew JS inside and out, this gem comes up.

String.prototype.doNothing = function() {
  return this;
};

alert(typeof 'foo'.doNothing()) // object
alert(typeof 'foo')             // string

http://jsfiddle.net/dJBmf/

This is breaking some things that expect a string, such as jQuery's .text(str) method.

Answer 1

To make sure you're always getting a string, trying using this code:

String.prototype.doNothing = function() {
    return this.toString();
};

alert(typeof 'foo'.doNothing())
alert(typeof 'foo')

In your original code, this is being returned as the string object and not the actual string.

Answer 2

Here's a thorough overview of the this keyword. Basically, JavaScript converts it into an object, if it wasn't one.

The following steps are performed when control enters the execution context for function code contained in function object F, a caller provided thisValue, and a caller provided argumentsList:

1. If the function code is strict code, set the ThisBinding to thisValue.
2. Else if thisValue is null or undefined, set the ThisBinding to the global object.
3. Else if Type(thisValue) is not Object, set the ThisBinding to ToObject(thisValue).
4. Else set the ThisBinding to thisValue

Same thing happens to Numbers and Booleans. A similar DoNothing function would return a type of object.

Answer 3

以strict模式运行代码以获得预期结果！

Answer 4

It's again the difference between string literals and strings, i believe? I once had a question answered here in SO: Property value of a String object in JavaScript

Answer 5

You could've also used the constructor property:

'foo'.constructor === String; //=>true
'foo'.doNothing().constructor === String; //=>true

See also this SO question and this jsFiddle

If String.prototype.doNothing() breaks stuff expecting a string value, I would use return String(this) or this.toString() ( this.valueOf() also works here) indeed.

Answer 6

尝试以下方法：

return this + '';

Answer 7

To get a better understanding of what's going on try using console logs, like so:

String.prototype.doNothing = function() {
    console.log(this);
    return this;
};

console.log(typeof 'foo'.doNothing());
console.log(typeof 'foo');

These are the results I get in Firefox:

foo { 0="f", 1="o", more...}
object
string

So it seems in the prototype a string is represented as an object/an array of characters (which does make sense.)

Whether or not you should use toString (as suggested by McHerbie) or casting as a type String (as suggested by mellamokb) depends, in my opinion, on what you plan to do with this value. I would personally lean toward casting it as a String.

Answer 8

function print() {
  let str = this;
  console.log(str);
}

String.prototype.print = print;

let a = "hello";
 a.print();//output:"hello"

Answer 9

Try return String(this); instead of return this;