Modifying a variable within an arrow function does not result in the variable being changed when checked outside of the arrow

Question

Modifying a variable within an arrow function does not result in the variable being changed when checked outside of the arrow

I am currently developing an application with Angular and Typescript where I need to update the value of a variable inside a function. To retrieve the data required, I'm utilizing a service. Below is the code snippet for reference:

 let isDataAvailable: boolean = false;

  onChangeValue(list: List) {
     this.someService.getData(list).subscribe(
        item => {
        if(item.value === 1) {
            isDataAvailable = true;
          }
       }); 
   console.log(isDataAvailable);          // The value still remains false at this point  
  }

javascript angular typescript arrow-functions

Answer 1

Answer №1

Hello there!

The reason for this behavior lies in the asynchronous nature of your service call. The browser does not wait for the service to emit a value, which is then handled in the subscription block.

isDataAvailable = false;

onChangeData(data: Data) {
    this.dataService.fetchData(data).subscribe(response => {
        if (response.value === 1) {
            this.isDataAvailable = true;
            this.updateCallback();
        }
    });   
}

updateCallback() {
    console.log(this.isDataAvailable); // true
}

You need to handle it in a similar manner as shown above. Hopefully you understand now (you can also search about javascript event loop for more insights).

Answer 2

Hello there!

The reason for this behavior lies in the asynchronous nature of your service call. The browser does not wait for the service to emit a value, which is then handled in the subscription block.

isDataAvailable = false;

onChangeData(data: Data) {
    this.dataService.fetchData(data).subscribe(response => {
        if (response.value === 1) {
            this.isDataAvailable = true;
            this.updateCallback();
        }
    });   
}

updateCallback() {
    console.log(this.isDataAvailable); // true
}

You need to handle it in a similar manner as shown above. Hopefully you understand now (you can also search about javascript event loop for more insights).

Answer 3

Answer №2

Given that Javascript operates asynchronously, the value will only update once the getData service call is completed.

In this scenario, the console output occurs before the getData operation takes place.

To address this issue, you have the option to utilize promises or callbacks.

See an example below:

isValue:boolean = false;

onChangeValue(list: List) { 
   onDataRecieved((res)=>{
     console.log(this.isValue);
   });    
}       

onDataRecieved(cb){
 this.someServive.getData(list).subscribe(
      item => {
      if(item.value === 1) {
         this.isValue = true;
         cb(true);
      }
   });
}

Answer 4

Given that Javascript operates asynchronously, the value will only update once the getData service call is completed.

In this scenario, the console output occurs before the getData operation takes place.

To address this issue, you have the option to utilize promises or callbacks.

See an example below:

isValue:boolean = false;

onChangeValue(list: List) { 
   onDataRecieved((res)=>{
     console.log(this.isValue);
   });    
}       

onDataRecieved(cb){
 this.someServive.getData(list).subscribe(
      item => {
      if(item.value === 1) {
         this.isValue = true;
         cb(true);
      }
   });
}

Modifying a variable within an arrow function does not result in the variable being changed when checked outside of the arrow

Answer №1

Answer №2

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