So I'm trying to call a method in a TypeScript class from within an array's ForEach loop. However, it seems that I can't figure out how to scope in the right 'this' for the parent class.

What I want to do is call the getFeatureAmount method from the survey.answerKey.q2SelectedValues.forEach(function(value)){ ... }); like this:

export class CalculationService {
        private _baseRate: BaseRate;
        private _subtotalPlatform: number = 0; 

        constructor(){
            this._baseRate = new BaseRate(125, 60);
        };

       //This is the method I'm trying to call
        private getFeatureAmount = (value: string, sub: number): number => {
            return sub += parseInt(value) * this._baseRate.local; 
        }

        public calculate(survey: Survey){

        let subtotal_ui: number = 0;
        subtotal_ui = (parseInt(survey.answerKey.q1SelectedValues[0]) * 5);

        survey.answerKey.q2SelectedValues.forEach(function(value){
            subtotal_ui = this.getFeatureAmount(value, subtotal_ui); //ERROR HERE. 'this' is undefined
        });

        return subtotal_ui + this._subtotalPlatform;
    }
}

But I get that 'this' is undefined and can't find the getFeatureAmount. As a temporary workaround, I have to call getFeatureAmount as a callback function.

private getFeatureAmount = (value: string): number => {
            return this._subtotalPlatform += parseInt(value) * this._baseRate.local; 
        }

survey.answerKey.q2SelectedValues.forEach(this.getFeatureAmount);

It's just not what i really wanted to do. So I'm wondering is there any way to do this with a lambda ()=>{}?

So I'm trying to call a method in a TypeScript class from within an array's ForEach loop. However, it seems that I can't figure out how to scope in the right 'this' for the parent class.

What I want to do is call the getFeatureAmount method from the survey.answerKey.q2SelectedValues.forEach(function(value)){ ... }); like this:

export class CalculationService {
        private _baseRate: BaseRate;
        private _subtotalPlatform: number = 0; 

        constructor(){
            this._baseRate = new BaseRate(125, 60);
        };

       //This is the method I'm trying to call
        private getFeatureAmount = (value: string, sub: number): number => {
            return sub += parseInt(value) * this._baseRate.local; 
        }

        public calculate(survey: Survey){

        let subtotal_ui: number = 0;
        subtotal_ui = (parseInt(survey.answerKey.q1SelectedValues[0]) * 5);

        survey.answerKey.q2SelectedValues.forEach(function(value){
            subtotal_ui = this.getFeatureAmount(value, subtotal_ui); //ERROR HERE. 'this' is undefined
        });

        return subtotal_ui + this._subtotalPlatform;
    }
}

But I get that 'this' is undefined and can't find the getFeatureAmount. As a temporary workaround, I have to call getFeatureAmount as a callback function.

private getFeatureAmount = (value: string): number => {
            return this._subtotalPlatform += parseInt(value) * this._baseRate.local; 
        }

survey.answerKey.q2SelectedValues.forEach(this.getFeatureAmount);

It's just not what i really wanted to do. So I'm wondering is there any way to do this with a lambda ()=>{}?

• javascript
• typescript
• foreach
• this

2 Answers 2

Try changing

survey.answerKey.q2SelectedValues.forEach(function(value){
   subtotal_ui = this.getFeatureAmount(value, subtotal_ui); //ERROR HERE. 'this' is undefined
})

to

survey.answerKey.q2SelectedValues.forEach((value) => {
   // now this will be refer to the instance of your CalculationService class
   subtotal_ui = this.getFeatureAmount(value, subtotal_ui);
});

var o = {
    one: function() { return this; },
    two: () => { return this; },
    three() { return this; },
    four() { return function () { return this; }; },
    five() { return () => { return this; }; }
}

o.one() === o
o.two() === window
o.three() === o
o.four()() === window
o.five()() === o

Don't declare methods with lambda syntax, since this will not be the object/class. Return or use lambda functions as arguments, if you want this to be the containing class.