how to create a js prototype class in external file

Question

I created an external .js file with prototype:

function Logger() {
    var log = new Array("");
}

Logger.prototype.AddLine = function (value) {
    if (value) {
        Logger.log.push("\t" + value);
    }
}

Logger.prototype.ReadLog = function () {
    return this.log.join("");
}

And now I try to use it on my page. I included my file in the header. And I have simple js:

$(document).ready(function () {

        var log = new Logger();
        log.AddLine("User entered the page");
});

Firebug error: TypeError: Logger.log is undefined [Break On This Error]
Logger.log.push("\t" + value);

Can anyone please explain why this is happening?

Yeah, but it's in document ready so it shouldn't matter anyway. — bobek
– bobek, Commented Feb 2, 2013 at 15:42
Can you check your assets if your external script has loaded? — Joseph
– Joseph, Commented Feb 2, 2013 at 15:43

Bergi · Accepted Answer · 2013-02-02 15:47:53Z

3

You are using four (!) different log things:

var log = new Logger(); is a (local) variable containing an new Logger instance
var log = new Array(""); is a (local) variable that is scoped to the constructor function it is declared in. You will not be able to access it from outside
Logger.log.… is a property on the Logger function object. There is nothing, so this line will throw the error.
this.log.… is what you actually want, a property on the current instance.

So change your script to this:

function Logger() {
    this.log = [""]; // not sure why not just an empty array
}

Logger.prototype.AddLine = function (value) {
    if (value) {
        this.log.push("\t" + value);
    }
};

Logger.prototype.readLog = function () {
    return this.log.join("");
};

answered Feb 2, 2013 at 15:47

Bergi

671k162 gold badges1k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

Comments

pimvdb · Accepted Answer · 2013-02-02 15:44:13Z

2

To set log on the Logger instance to an empty array, use this.log = [], not var log. The latter creates a private variable, which you can't access outside the function.

Also, Logger.log means: access the log property on the Logger function. Here you want the instance instead, so use this.log (like you did in ReadLog).

answered Feb 2, 2013 at 15:44

pimvdb

155k80 gold badges313 silver badges358 bronze badges

Collectives™ on Stack Overflow

how to create a js prototype class in external file

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related