Encountering issues with reassigning variables in Angular resulting in null or undefined values

Currently, I am dealing with a test array where the initial value is set to null.

In my function, I am making some modifications to the test array. However, even though I am reassigning values to it, the console still shows it as a null or undefined array.

abcd(){
    this.dataService.getAirport().subscribe(
      (data) => {
        this.airportData = data.data.data.metaDataRows;
        this.countryData = data.data.data.metaDataFields[0].column;
         const airConNames = this.countryData.values;
         this.test = [];
         this.test.push({name:'Select a Country', id:'0'});
         //this.test = [{name:'Select a Country', id:'0'}];
         console.log(this.test);
         airConNames.forEach(function(entry) {
           //console.log(entry.name);
           //console.log(entry.country_id);
          this.test = [{name : entry.name, id : entry.country_id}];
         });
        console.log(this.test); // this is null
      },
      (error) => {
        this.dataService.handleServiceError(error.message, this.TAG);
      }
    );
      console.log(this.test); //this is null
 }

Despite my efforts, the console continues to display null for the test array.

I am seeking assistance in identifying where I may have made an error in my code.

Answer №1

There are a couple of things to consider within the foreach loop. Instead of assigning this.test to a new array each time, it would be more efficient to perform a this.test.push() operation or utilize rest arguments by doing something like

this.test = [{ ... }, ...this.test];
(depending on whether you want to add elements at the beginning (unshift) or end (push).

Furthermore, the context of this may not be what you anticipate when using

airConNames.forEach(function(entry) { ... })
. It might be beneficial to switch to arrow function syntax such as
airConNames.forEach((entry) => {

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

Arrow functions have a concise syntax compared to regular functions and do not have their own this, arguments, super, or new.target.

If this.test is unexpectedly null after the foreach, it could be worth trying the following approach:

this.test = airConNames.map(entry => ({ name: entry.name, id: entry.country_id }));
this.test.unshift({ name: 'Select a Country', id:'0' });

Answer №2

 airConNames.forEach(function(entry) {
       //console.log(entry.name);
       //console.log(entry.country_id);
       // push new values into your test array:
      this.test.push({name : entry.name, id : entry.country_id});
     });
    console.log(this.test); // this is null
  },

The scope inside the forEach function affects the value of this.test when accessed outside the loop. This discrepancy occurs because the context changes within the loop compared to outside where the console.log statement is executed.

To ensure consistency with the variable declared above, you can use the arrow function notation => instead. Arrow functions have a concise syntax and do not create their own this, making them suitable for non-method functions. They are not intended for use as constructors. Example:

airConNames.forEach((entry) => {
   //console.log(entry.name);
    //console.log(entry.country_id);
// push new values into your test array:
  this.test.push({name : entry.name, id : entry.country_id});

});

For more information on arrow functions, visit: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

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