TypeError: "this is undefined" When using forEach on member array

Question

I have a class marketData:

export class marketData {
    id: number;
    denomCount: number;
    itemCount: number;
    conversionRates: Array<number> = [];
    conversionRateToLowest: Array<number> = [];
    itemPrices: Array<Array<number>> = [];
}

I would like to define a member function for it, validate(), that checks that the values of the class are set properly. I wrote part of this method to check itemPrices:

this.itemPrices.forEach(function(this, prices){
          if(prices.length != this.itemCount){
              // handle error 
          });

However, the above gives me a ERROR TypeError: "this is undefined". I got the exact same error trying to check itemPrices in this manner:

this.itemPrices.forEach(function(prices){
          if(prices.length != this.itemCount){
              // handle error
          });

i.e. without the this in the function parameters.

What is the correct way to access the itemCount member variable?

Manish · Accepted Answer · 2019-05-03 03:03:20Z

3

Its because of the the way you are using forEach. Use Arrow Functions instead like below. And its recommended that when using typescript classes always use arrow functions else you will keep running into this problem.

this.itemPrices.forEach((prices) =>{
          if(prices.length != this.itemCount){
              // handle error
            }
});

answered May 3, 2019 at 3:03

Manish

5,0963 gold badges33 silver badges47 bronze badges

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

Comments

wentjun · Accepted Answer · 2019-05-03 03:33:25Z

1

You can make use of ES6's arrow functions to access this. Anyways, do read this up to udnerstand to context of this when it comes to JavaScript and using them within OOP in JS.

this.itemPrices.forEach((prices: number) => {
  if (prices.length != this.itemCount){
    // handle error 
  });
});

edited May 3, 2019 at 3:33

answered May 3, 2019 at 3:03

wentjun

42.8k10 gold badges108 silver badges116 bronze badges

Comments

Saeed · Accepted Answer · 2021-02-09 11:42:11Z

1

The value of this in the function is not the same as it is inside the class. You can either use fat arrow function or use bind(this) to bind the function to the class.

this.itemPrices.forEach(function(prices){
          if(prices.length != this.itemCount){
              // handle error
          }).bind(this);

edited Feb 9, 2021 at 11:42

Saeed

3,5466 gold badges40 silver badges58 bronze badges

answered May 3, 2019 at 3:00

Sachin Gupta

5,3493 gold badges19 silver badges33 bronze badges

Collectives™ on Stack Overflow

TypeError: "this is undefined" When using forEach on member array

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related