javascript - Typescript undefined is not an object when pushing to array - Stack Overflow

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I am trying to push to an array but it keeps giving me this error:

Uncaught (in promise): TypeError: Cannot read property 'push' of undefined
TypeError: Cannot read property 'push' of undefined

here is the source where I am simply adding to an array. I take the obsevable from the firestore collection and loop through it. Why is it giving me this error when the array of category objects is right there. categories: Category[];

transactions: Observable<Transaction[]>;
categories: Category[];

private organizeData() {
    let category: Category;

    this.transactions.forEach(v => {
      for (let i = 0; i < v.length; i++) {
        category = {name: v[i].category, totalSpent: v[i].amount};
        this.categories.push(category);
      }
    });
  }

I am trying to push to an array but it keeps giving me this error:

Uncaught (in promise): TypeError: Cannot read property 'push' of undefined
TypeError: Cannot read property 'push' of undefined

here is the source where I am simply adding to an array. I take the obsevable from the firestore collection and loop through it. Why is it giving me this error when the array of category objects is right there. categories: Category[];

transactions: Observable<Transaction[]>;
categories: Category[];

private organizeData() {
    let category: Category;

    this.transactions.forEach(v => {
      for (let i = 0; i < v.length; i++) {
        category = {name: v[i].category, totalSpent: v[i].amount};
        this.categories.push(category);
      }
    });
  }

• javascript
• typescript

Share Improve this question Follow edited Jun 21, 2018 at 17:10 Frank van Puffelen 600k8585 gold badges890890 silver badges860860 bronze badges asked Jun 21, 2018 at 15:41 Zach StarnesZach Starnes 3,19899 gold badges4242 silver badges6666 bronze badges

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2 Answers 2

Sorted by: Reset to default 8

categories: Category[] is just the declaration of an array, you need to actually create a new array and assign it to the field:

categories: Category[] = []

You are not initiating your array categories so it will return undefined. Try:

categories: Category[] = [];

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