Javascript promise failing to deliver

Question

Javascript promise failing to deliver

As a beginner in the world of JavaScript development, I am excited to be part of the stackoverflow community and have already gained valuable insights from reading various posts.

Currently, I am facing an issue where I need to load a file, but due to its asynchronous nature, the code is not waiting for it to finish loading. While I attempted using a Promise, it seems to still not wait for the file to load before moving on. Interestingly, it does wait when there's no file present. This behavior is counterintuitive as it should prioritize waiting for the file to load first and then proceed with executing the next function that requires the data from the file. Can someone please shed some light on why my promise implementation is not working? Should I consider using a callback instead?

Below is the snippet of the code causing me trouble:

// ...

var p = new Promise(function (resolve, reject) {
    let x = getTheData(); // async data but program doesn’t wait.
    if (!x) { //if no data, I want to wait?
        resolve('success');
        console.log('success');
    }else {
        reject('failure');
        console.log('failure');
    }
});

p.then(function () {
    let y = getNewFunctionThatNeedsTheAboveDataToWork();
    res.send('y is not working ' + y);
}).catch(function(){
    console.log('error');
});

return p;

I'm feeling completely lost at this point, any guidance would be greatly appreciated.

javascript json asynchronous callback promise

Answer 1

Answer №1

In this scenario, the issue arises because getData operates asynchronously, requiring a method to determine when it completes its task. This implies that getData must either return a promise (the modern approach) or accept a callback as an argument (the traditional approach). Below is an illustration of how promises can be utilized with getData to fetch data through a web call using pure JavaScript:

var getData = function() {
  var p = new Promise(function (resolve, reject) {
    var xhr = new XMLHttpRequest();
    xhr.addEventListener("readystatechange", function () {
      if (this.readyState === 4 && this.status != 200) {
          console.log('Data request returned an error');
          reject('failure');
      } else if (this.readyState === 4 && this.status === 200) {
          resolve(this.responseText);
      }
    });
    xhr.open("GET", "https://httpbin.org/get", true); // true = async
    xhr.send();
  })
  return(p)
}

This function can then be used as follows:

getData().then(function(d) {
  console.log(d);
  // perform actions with d
})

Alternatively, achieving the same result can be accomplished using a callback:

var getData2 = function (cb) {
  var xhr = new XMLHttpRequest();
  xhr.addEventListener("readystatechange", function () {
    if (this.readyState === 4 && this.status != 200) {
        console.log('Data request returned an error');
        cb(null, "error");
    } else if (this.readyState === 4 && this.status === 200) {
        cb(this.responseText);
    }
  });
  xhr.open("GET", "https://httpbin.org/get", true);
  xhr.send();
}

getData2(function(d, e) {
  if(e) {
    console.log("oops.  there was an error: ", e);
  } else {
    console.log(d);
  }
})

If you are unable to modify the getData function and it utilizes a callback structure, you can transform it into a promise-based function. Here's one approach:

var promiseMe = new Promise(function (resolve, reject) {
  getData2(function(d, e) {
    if(e) {
      reject(e)
    } else {
      resolve(d)
    }
  })
})

promiseMe.then(function(d) {
  console.log(d)
})

EDIT

The previously mentioned methods are intended for browser usage. To implement a similar solution in node.js, consider the following example:

http = require('http');

var getData = function() {
  var p = new Promise(function (resolve, reject) {
    var data = '';
    http.get('http://httpbin.org/get', function(res) {
        res.on('data', (chunk) => {
            data += chunk;
        });

        res.on('end', () => {
            resolve(data);
        });
    })
  })
  return(p)
}
getData().then(function(d) {
    console.log(d)
})

These examples should help clarify the process.

Answer 2

In this scenario, the issue arises because getData operates asynchronously, requiring a method to determine when it completes its task. This implies that getData must either return a promise (the modern approach) or accept a callback as an argument (the traditional approach). Below is an illustration of how promises can be utilized with getData to fetch data through a web call using pure JavaScript:

var getData = function() {
  var p = new Promise(function (resolve, reject) {
    var xhr = new XMLHttpRequest();
    xhr.addEventListener("readystatechange", function () {
      if (this.readyState === 4 && this.status != 200) {
          console.log('Data request returned an error');
          reject('failure');
      } else if (this.readyState === 4 && this.status === 200) {
          resolve(this.responseText);
      }
    });
    xhr.open("GET", "https://httpbin.org/get", true); // true = async
    xhr.send();
  })
  return(p)
}

This function can then be used as follows:

getData().then(function(d) {
  console.log(d);
  // perform actions with d
})

Alternatively, achieving the same result can be accomplished using a callback:

var getData2 = function (cb) {
  var xhr = new XMLHttpRequest();
  xhr.addEventListener("readystatechange", function () {
    if (this.readyState === 4 && this.status != 200) {
        console.log('Data request returned an error');
        cb(null, "error");
    } else if (this.readyState === 4 && this.status === 200) {
        cb(this.responseText);
    }
  });
  xhr.open("GET", "https://httpbin.org/get", true);
  xhr.send();
}

getData2(function(d, e) {
  if(e) {
    console.log("oops.  there was an error: ", e);
  } else {
    console.log(d);
  }
})

If you are unable to modify the getData function and it utilizes a callback structure, you can transform it into a promise-based function. Here's one approach:

var promiseMe = new Promise(function (resolve, reject) {
  getData2(function(d, e) {
    if(e) {
      reject(e)
    } else {
      resolve(d)
    }
  })
})

promiseMe.then(function(d) {
  console.log(d)
})

EDIT

The previously mentioned methods are intended for browser usage. To implement a similar solution in node.js, consider the following example:

http = require('http');

var getData = function() {
  var p = new Promise(function (resolve, reject) {
    var data = '';
    http.get('http://httpbin.org/get', function(res) {
        res.on('data', (chunk) => {
            data += chunk;
        });

        res.on('end', () => {
            resolve(data);
        });
    })
  })
  return(p)
}
getData().then(function(d) {
    console.log(d)
})

These examples should help clarify the process.

Javascript promise failing to deliver

Answer №1

Similar questions

Customize Material-UI FAB Hover Color

Create compelling visual representations by utilizing Google charts to draw customized charts based on variable

Using TCL/expect to create a log file in XML or JSON format

Adjust the hue of a circle based on whether the user's position falls within its designated range

It appears that the SignalR proxy is not defined

When working with NodeJS and an HTML form, I encountered an issue where the 'Endpoint'

Change UL to a JSON format

PHP and AJAX collaboration encounters issue with returning JSON data

Loading data from JSON files into Postgres tables

Animate a div component sliding out while seamlessly introducing a new one using React

Sending an array of elements to an API using Python

error: local server did not return any data

Could one harness the power of SO's script for adding color to code within questions?

What is the best way to showcase a set of paired arrays as key-value pairs?

The compatibility between cross-fetch and React Native is currently not supported

What is the best way to retrieve a FireStore document ID from an object?

Relocating JavaScript scripts to an external file on a webpage served by Node.js' Express

Unexpected behavior: JQuery Ajax request not displaying Json object following recent update to JQuery version 1.10.2

What is the best source for JSON serialization when using Ktor?

Learn the trick to make this floating icon descend gracefully and stick around!