为什么 String.Prototype 替换在嵌套函数中不起作用？

Question

I have declared in same script file the following sub-string replace function :

String.prototype.replaceAt = function(index, character) {
    index = parseInt(index, 10);
    return this.substr(0, index) + character + this.substr(index + character.length);
}

If I use this function in the main script file (for example right after its declaration), it works properly with string output.

If I use this function inside nested functions, more exactly I have a function inside another function and I call "replaceAt" inside the second function, it doesn't work and it truncates all characters after the "index" specified in "replaceAt". I also specify that this is a content script in a Chrome extension.

Example (works okay outside functions, in main file) :

var h = '000000';
h = h.replaceAt(3, "1");
console.log(h);

Example (truncates everything after "index") :

function do_lut() {
    temp = '000000000000000';

    function nfu_change(e, i) {
        if (e.checked) {
            if (temp != null) {
                console.log(i + " - " + temp);
                temp = temp.replaceAt(i, "1");
            } else { temp = '000000000000000'.replaceAt(i, "1"); } 
                       }
        else { if(temp!=null) {temp = temp.replaceAt(i,"0");} else {temp = new String('000000000000000');} }
        console.log(i+" - "+temp);
        }
    }

 for(i=0;i<15;i++) 
 {
  nfu[i] = document.createElement('INPUT');
  nfu[i].type = 'checkbox';
  nfu[i].id = Math.floor((Math.random() * 100000000) + 1).toString();
  nfu[i].name = i.toString();
  nfu[i].onchange = function(e) {nfu_change(e.target,e.target.name);}
 }
}

The above will create a list of checkbox inputs and when user checks/unchecks a box, the respective index (corresponding to the input in the list) will be changed to either "0" or "1" true/false in "temp" which will be overwritten in a cookie. So, "nfu_change" is called conditionally on change of checkbox status.

Answer 1

My hypothesis is that this doesn't mean what you think it means. Run your code thru a debugger and see what value this takes on in the places where your function does and does not do what you expect.

---

I think that the above hypothesis is wrong.

I was able to reproduce your problem by calling your replaceAt function with a string for the index.

String.prototype.replaceAt = function (index, character) {
    return this.substr(0, index) + character + this.substr(index +  character.length);
}

alert("abc".replaceAt(1, "B")); // aBc
alert("abc".replaceAt("1", "B")); //aB

Here's the solution:

String.prototype.replaceAt = function (index, character) {
    index = parseInt(index, 10);
    return this.substr(0, index) + character + this.substr(index + character.length);
}

alert("abc".replaceAt(1, "B")); // aBc
alert("abc".replaceAt("1", "B")); //aBc