Ionic 3: Struggling to Access Promise Value Outside Function

I've encountered an issue where I am unable to retrieve the value of a promise outside of my function. It keeps returning undefined.

Here's what I have attempted so far:

Home.ts

class Home{

dd:any;

constructor(public dbHelpr:DbHelperProvider){

}

getData(){

   this.dbHelpr.getRecordById("51").then(function(res:any){

               dd = res
                console.log("dd in "+dd);

              }); 

              console.log("dd out "+dd);
           }

}

DbHelperProvider.ts

getRecordById(_id){

    return new Promise(resolve => {
      this.db.get(_id).then(function (doc) {

        resolve(doc);
      }).catch(function (err) {
        console.log(err);
        resolve("fail");
      });
    })

  }

This is what I found in my log:

dd inside abcdef

dd outside undefined

Can anyone offer a solution to this problem?

Thank you in advance for your help!

angulartypescriptionic-frameworkionic2ionic3

Answer №1

Due to the asynchronous nature of promises, the statement console.log("dd out "+dd); gets executed before console.log("dd in "+dd);. This is why you are receiving undefined for dd out.

In order to resolve this issue, you need to assign the value of the variable doc to a global variable within the .then method. Here is an example:

this.db.get(_id).then(function (doc) {

  resolve(doc);
  // assign the value to a global variable here.
})

Answer №2

The issue lies in the absence of an arrow function. When using a common function within a callback, it does not provide access to variables outside of it. An arrow function is required for this purpose. Additionally, the usage of this is necessary to access the dd property of the class.

getData(){
  this.dbHelpr.getRecordById("51").then((res:any) => {
    console.log("dd in " + this.dd); // displays undefined
    this.dd = res;
    console.log("dd out " + this.dd); // displays the value
  });
}

In addition, there is a mention by VicJordan regarding the async. As the second console.log statement is placed outside of it, it is executed before the first one. Consequently, since no value is initialized for the dd property, it remains as undefined.

I hope you find this information helpful.

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