CODEWITHSUNDEEP
javascriptprototype
mikelee54
mikelee54

Reputation: 29

Inheriting prototype using .call(this)

(function() {

    LoggerBase.prototype.output = function(message) {
        console.log('LoggerBase: ' + message);
    };

    function BookAppLogger() {

        LoggerBase.call(this);

        this.logBook = function(book) {
            console.log('Book: ' + book.title);
        }
    }

    BookAppLogger.prototype = Object.create(LoggerBase.prototype);

}());

In this code the BookAppLogger inherits the prototypes of the LoggerBase object, I think that is clear from the last statement. What I don't understand is the purpose of the LoggerBase.call(this) statement. What does this line do and why is it neccessary?

Upvotes: 1

Views: 46

Answers (2)

Andrew Skirrow
Andrew Skirrow

Reputation: 3451

Its just calling the base class constructor

LoggerBase is a function object and the call method of a function can be used to call that function with a specific this value. Calling LoggerBase directly would result in this being the global object, which in a browser is the window object.

function LoggerBase() {
     console.log(this);
}

function BookAppLogger() {

    LoggerBase.call(this); // will print the book logger
    LoggerBase();          // will print the global object
}

BookAppLogger.prototype = Object.create(LoggerBase.prototype);

Upvotes: 0

Kiran Reddy
Kiran Reddy

Reputation: 556

BookAppLogger.prototype = Object.create(LoggerBase.prototype);

will only add LoggerBase.prototype functions to BookAppLogger.prototype but you cannot inherit the functions/properties that are written inside LoggerBase function. For example if LoggerBase is something like

function LoggerBase () {
    this.fname = "First";
    this.lname = "Last";

    this.fullname = function(){
        return this.fname + " " + this.lname;
    }
}

LoggerBase.prototype.somefunction = function(){}

If you do not write LoggerBase.call(this) inside BookAppLogger, then only only LoggerBase somefunction is inherited but not fname, lname, fullname

Upvotes: 1

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