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

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