JavaScript - doing something wrong with a String prototype function

Question

I'm trying to create a custom string method.

I'm not sure how to get the value of the string, that the method is attached to, passed to the function and back.

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);

Thanks for the answers on "this". But with my complex real functions, what is the syntax to get "this" to be processed by everything I'm doing in the function? — user3201867
– user3201867, Commented Jan 16, 2014 at 10:26
Thanks for everyone's help. The scales have been lifted from my eyes. I now completely grok the nature of "this"! (Pretty much.) Unfortunately I can't vote any of you up as I have "no reputation". — user3201867
– user3201867, Commented Jan 16, 2014 at 15:56

Allan Kimmer Jensen · Accepted Answer · 2014-01-16 11:45:33Z

3

You need to use this like here:

function testPrototyping() {
    var text = this;

    text = text.trim();

    text + " reason why this is not working";

    return text;
}

String.prototype.testMethod = testPrototyping;

"StackOverflow".testMethod()

The Keyword this inside a Prototype Method Refers to a Constructor instance. You can read more about "this" here.

edited Jan 16, 2014 at 11:45

answered Jan 16, 2014 at 10:17

Allan Kimmer Jensen

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3 Comments

user3201867 Over a year ago

For the "this" produced by my method call here... How do I assign the string value of "this" to a variable inside the function. Then do a string.slice() on the variable. Then assign the result of that back to "this". Then return "this" to my method call?

Allan Kimmer Jensen Over a year ago

I updated my example, you can just assign this to a variable, and then return it when you are done manipaluting it.

user3201867 Over a year ago

Thanks -- that's the practical solution I need. I'm curious though theoretically whether "return text" here is secretly assigning "text" as a value of "this" to be passed back to the calling method...

basilikum · Accepted Answer · 2014-01-16 10:18:00Z

2

If you want to refer to the current string, you can use the this keyword:

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

You can however not assign anything to this. So this += "..." wouldn't work.

answered Jan 16, 2014 at 10:18

basilikum

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2 Comments

user3201867 Over a year ago

This is my question now... how to assign everything to "this" when I can't assign anything to "this" :)

basilikum Over a year ago

Well why do you want to assign to this. Your example indicates that you want to assign the result to a new variable myVar2: myVar2 = myVar.testMethod();. Or do you want to change the value of myVar as well? Because this is not possible without directly referring to this variable by name.

Tibos · Accepted Answer · 2014-01-16 10:36:40Z

0

The Object wrapper testPrototyping will be called on is available via this inside testPrototyping. The actual underlying string of a String Object wrapper is available via the valueOf() method:

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 
*/

edited Jan 16, 2014 at 10:36

answered Jan 16, 2014 at 10:24

Tibos

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1 Comment

Tibos Over a year ago

I found the source of unicorns

Sebastien C. · Accepted Answer · 2014-01-16 12:45:56Z

0

You can't do exactly what you want because strings are immutable in javascript. It means that you can't change a string without creating a new one.

Depending on what you really need to achieve, you can use another object to "hide" the real string :

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"

answered Jan 16, 2014 at 12:45

Sebastien C.

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1 Comment

user3201867 Over a year ago

Thank you. I understand now that "this" is a read-only object/thingy, but that values can be assigned to properties of it.

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