Uncaught TypeError: Unable to access 'product' property as it is undefined

I am facing an issue with a private variable named product that I declare in the constructor. Inside the ngOnInit function, I am using array.forEach() to loop through some sections from a service and assign a value to the product variable. Even though console.log displays the correct value, I keep getting an error stating 'Cannot Read Properties of undefined' related to product. Can someone please help me figure out what mistake I might be making?

private product: string;

constructor(private service: Service) {
  this.product = "";
}

ngOnInit() {
  this.service.sections.forEach(function (value) {

  if (this.product) {
    this.product = value.Product.ShortName;
  } else {
    this.product = "";
  }
  console.log(value.Product.ShortName);
});
}

Answer №1

Transform the function (value) {...} into an arrow function

Arrow functions lack their own bindings to this, arguments or super, and are unsuitable for use as methods.

this.service.sections.forEach((section) => { 
  if (this.product) {
    this.product = section.Product.ShortName;
  } else {
    this.product = "";
  }
  console.log(section.Product.ShortName);
});

Answer №2

@MikeOne, please make sure to mark this point as important.

The issue you are facing is likely due to a context error, specifically related to the product class not being properly referenced using this.

In a traditional JavaScript function, the reference to the function context should be explicit. Therefore, assigning it to a global accessor that can be easily accessed should resolve the problem.

   ngOnInit() {
      var self = this
      this.service.sections.forEach(function (value) {

      if (self.product) {
         self.product = value?.Product?.ShortName || "";
      } else {
         self.product = "";
      }
      console.log(value.Product.ShortName);
   });
 }

As a further tip for handling potential null values in callbacks, keep in mind that JavaScript will throw an exception if you try to access a property of a null object, such as Product.

To prevent these exceptions, consider using the Elvis operator ?, like so:

Therefore, your assignment should follow this pattern:

this.product = value?.Product?.ShortName || null;

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