How do you get data from a callback function?

How do you get data from a callback function?

Here’s the code: var pdfText = require(‘pdf-text’); var pathToPdf = “PDF FILE NAME”; var fs = require(‘fs’); var buffer = fs. readFileSync(pathToPdf); var output; pdfText(buffer, function(err, chunks){ if (err){ console. dir(err); return; } console.

How do you return a promise from a callback?

To convert a callback into a promise, you need to return a promise. You run the code with the callback inside the promise. const readFilePromise = () => { return new Promise((resolve, reject) => { fs. readFile(filePath, options, (err, data) => { // }) }) }

What does a callback return?

Simply put: A callback is a function that is to be executed after another function has finished executing — hence the name ‘call back’. More complexly put: In JavaScript, functions are objects. Because of this, functions can take functions as arguments, and can be returned by other functions.

How do I access my promise results?

To access the value of a promise, call the then() method on the promise, e.g. p. then(value => console. log(value)) . The then() method takes a function, which is passed the resolved value as a parameter.

How do you return a value from promise?

If a handler function:

  1. returns a value, the promise returned by then gets resolved with the returned value as its value.
  2. doesn’t return anything, the promise returned by then gets resolved with an undefined value.
  3. throws an error, the promise returned by then gets rejected with the thrown error as its value.

How does a callback function return a value?

We create a new promise, an object that will be returned from our callback using the new Promise() function. We invoke a . then() function on our promise object which is an asynchronous function and passes our callback to that function. That callback function takes in two parameters, a resolve, and a reject.

How do you access the correct this inside a callback?

3 methods for accessing the correct this inside a callback

  1. Use an arrow function. JavaScript arrow functions were introduced in ECMAScript 6.
  2. Create another variable to store the this object.
  3. Explicitly bind this to an object.

How do you get a response body on Fetch?

Methods to get response body:

  1. text() – return the response as text,
  2. json() – parse the response as JSON object,
  3. formData() – return the response as FormData object ( multipart/form-data encoding, see the next chapter),
  4. blob() – return the response as Blob (binary data with type),

How do you pass a callback function in TypeScript?

Similar to JavaScript, to pass a function as a parameter in TypeScript, define a function expecting a parameter that will receive the callback function, then trigger the callback function inside the parent function.

How do I return a promise data?

function getPromise() { return new Promise(function(resolve, reject) { setTimeout(function() { resolve({ ‘country’: ‘INDIA’ }); }, 2000) }) } function getResult() { getPromise() . then(function(response) { return response; }) } let result = getResult(); console. log(result);

Does fetch return a promise?

Calling fetch() starts a request and returns a promise. When the request completes, the promise resolves to the response object. From the response object you can extract data in the format you need: JSON, raw text, Blob.

What is callback () in JavaScript?

A JavaScript callback is a function which is to be executed after another function has finished execution. A more formal definition would be – Any function that is passed as an argument to another function so that it can be executed in that other function is called as a callback function.

How do I retrieve data from Fetch?

Use the fetch() method to return a promise that resolves into a Response object. To get the actual data, you call one of the methods of the Response object e.g., text() or json() . These methods resolve into the actual data.