Javascript Object to get this of parent method

Question

In the following add method of myObj how can I get this inside map? In other words, this when wrapped to map, points to that anonymous function inside map. How can I get this there?

Note: Workarounds like creating a new variable temp_sumand adding and returning are not preferred. Because, I might have to do some tests inside them using the this keyword.

var myObj = {

    sum        : 0,
    toAdd      : [2,3,4],
    add        : function(){

        this.toAdd.map(function(num){
           this.sum += num //<-- How to get this.sum from here           
        })

       return this.sum;

    }


};

var m = Object.create(myObj);
var _sum = m.add();
document.getElementById("test").innerHTML = _sum;

@Vld, Just working around map because it looks clean. Looks like should use for loop. — X Pahadi
– X Pahadi, Commented Apr 4, 2015 at 17:21

adeneo · Accepted Answer · 2015-04-04 17:33:24Z

2

You could use bind

var myObj = {

    sum        : 0,
    toAdd      : [2,3,4],
    add        : function(){

        this.toAdd.map(function(num, index){
           this.sum += num;
        }.bind(this))

       return this.sum;
    }
};

or reduce

var myObj = {

    sum        : 0,
    toAdd      : [2,3,4],
    add        : function(){
        this.sum = this.toAdd.reduce(function(a,b){
           return a + b;
        });

        return this.sum;
    }
};

or a for loop

var myObj = {

    sum        : 0,
    toAdd      : [2,3,4],
    add        : function(){
        for (var i=0; i<this.toAdd.length; i++) {
            this.sum += this.toAdd[i];
        }

        return this.sum;
    }
};

edited Apr 4, 2015 at 17:33

answered Apr 4, 2015 at 17:21

adeneo

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2 Comments

X Pahadi Over a year ago

Excellent. One more thing. It is not necessary but can is there a way to get Index of the item inside map?

adeneo Over a year ago

Yes, map has three built in arguments, they are map(currentValue, index, array)

dfsq · Accepted Answer · 2015-04-04 17:31:50Z

1

Array.prototype.map method accepts optional argument: object value to be used as this. So your code will become as simple as:

add: function () {
    this.toAdd.map(function (num) {
        this.sum += num;      
    }, this);
    return this.sum;
}

answered Apr 4, 2015 at 17:31

dfsq

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