Understanding the method of recovering a nested promise

Question

Understanding the method of recovering a nested promise

I am facing an issue with returning the result parameter from the getColumn function. Despite my attempts, it keeps logging as undefined.

The connection function establishes a connection to a SQL database and retrieves a data set through a query.

Is there a way for me to effectively pass the variable up the promise chain?

getColumn = function(columnName, table) {
  sql.connect(config.properties)
    .then(result => {
      let request = new sql.Request();
      request.query("SELECT " + columnName + " FROM " + table)
      .then(result => {
          // trying to return this 'result' from the getColumn function
          return result
      }).catch(err => {
          // Error handling for queries
      })
    }).catch(err => {
      // Error handling for connections
    })
} // 

console.log(getColumn('username', 'Login'))

javascript promise

Answer 1

Answer №1

Initially, attempting to directly return a value from getColumn() is not feasible. The function's internal operations are asynchronous, meaning the final value cannot be determined until after getColumn() has completed its execution. As a result, getColumn() currently returns undefined due to lacking a direct return value. The existing return statement pertains to an asynchronous .then() handler rather than the core getColumn() method. Given the asynchronous nature of getColumn(), it is impossible to simply return the end result. Instead, employing a promise or callback system is necessary. Considering that promises are already utilized within the function, opting for a promise-based approach would be most suitable.

To address this issue, you can modify getColumn() to return a promise and subsequently handle it using either .then() or await.

In order to return a promise, it is essential to propagate the underlying promises:

const getColumn = function(columnName, table) {
  // return promise
  return sql.connect(config.properties).then(result => {
    let request = new sql.Request();
    // chain this promise onto prior promise
    return request.query("SELECT " + columnName + " FROM " + table);
  });
} // 

getColumn('username', 'Login').then(val => {
   console.log(val);
}).catch(err => {
   console.log(err);
});

Answer 2

Initially, attempting to directly return a value from getColumn() is not feasible. The function's internal operations are asynchronous, meaning the final value cannot be determined until after getColumn() has completed its execution. As a result, getColumn() currently returns undefined due to lacking a direct return value. The existing return statement pertains to an asynchronous .then() handler rather than the core getColumn() method. Given the asynchronous nature of getColumn(), it is impossible to simply return the end result. Instead, employing a promise or callback system is necessary. Considering that promises are already utilized within the function, opting for a promise-based approach would be most suitable.

To address this issue, you can modify getColumn() to return a promise and subsequently handle it using either .then() or await.

In order to return a promise, it is essential to propagate the underlying promises:

const getColumn = function(columnName, table) {
  // return promise
  return sql.connect(config.properties).then(result => {
    let request = new sql.Request();
    // chain this promise onto prior promise
    return request.query("SELECT " + columnName + " FROM " + table);
  });
} // 

getColumn('username', 'Login').then(val => {
   console.log(val);
}).catch(err => {
   console.log(err);
});

Understanding the method of recovering a nested promise

Answer №1

Similar questions

Integrating webpack with kafka-node for seamless communication between front

Can you please guide me on how to convert pug (jade) to html using npm scripts?

Is the variable empty outside of the subscribe block after it's been assigned?

Is it possible to extract the body from the post request using req.body.item?

Enhancing presentation with a multitude of pictures

Is it possible to use both interfaces and string union types in TypeScript?

Using JavaScript to launch a new window with an array of parameters

Determine the present height of the current class and substitute it with another class that has the same

Unexpected value detected in D3 for translate function, refusing to accept variable

Utilizing the power of JavaScript within CSS styling

Clicking on a DIV using jQuery, with anchor elements

What is the best way to ensure a table is responsive while maintaining a fixed header? This would involve the table scrolling when it reaches the maximum viewpoint, while also keeping

Encountering issue while starting npm: expressjs server facing problem with MongoDB

Why is it difficult to display data fetched through getJSON?

The URL is being modified, yet the page remains static in the React application

Why is it that consolidating all my jQuery plugins into one file is ineffective?

AngularJS can be used to display a webpage

What is the method for toggling background URLs?

Issue: Incorrect parameters for executing the MySQL statement

Transform the characters within a string into corresponding numerical values, calculate the total sum, and finally display both the sum and the original string