Promise, async/await (प्रतिज्ञा, असिंक/प्रतीक्षा)

Imagine you are a popular book writer, and you are planning to release a new book on a certain day. Readers who have an interest in this book are adding this book to their wishlist and are notified when published or even if the release day got postponed too. On the release day, everyone gets notified and can buy the book making all parties happy. This is a real-life analogy that happens in programming.

कल्पना गर्नुहोस् कि तपाईं एक लोकप्रिय पुस्तक लेखक हुनुहुन्छ, र तपाईं एक निश्चित दिनमा नयाँ पुस्तक जारी गर्ने योजना बनाउँदै हुनुहुन्छ। यस पुस्तकमा रुचि राख्ने पाठकहरूले यस पुस्तकलाई आफ्नो इच्छासूचीमा थपिरहेका छन् र प्रकाशित हुँदा वा प्रकाशन को दिन पनि स्थगित भएको अवस्थामा सूचित गरिन्छ। रिलीजको दिन, सबैलाई सूचित गरिन्छ र सबै पक्षलाई खुसी पार्ने पुस्तक किन्न सकिन्छ। यो एक वास्तविक जीवन सादृश्य हो जुन प्रोग्रामिंगमा हुन्छ।

1. A "producing code" is something that takes time and accomplishes something. Here it's a book writer.

   "producing code" भनेको समय लाग्ने र कुनै कुरा पूरा गर्ने कुरा हो। यहाँ यो एक पुस्तक लेखक हो।

2. A "consuming code" is someone who consumes the "producing code" once it's ready. In this case, it's a "reader".

   "consuming code" ले "उत्पादन कोड" एक पटक तयार भएपछि उपभोग गर्दछ। यस मामला मा, यो एक "पाठक" हो।

3. The linkage between the "producing code" and the "consuming code" can be called a promise as it assures getting the results from the "producing code" to the "consuming code".

"producing code" र "consuming code" बीचको सम्बन्धलाई प्रतिज्ञा भन्न सकिन्छ किनकि यसले "producing code" बाट "consuming code" मा परिणामहरू प्राप्त गर्ने आश्वासन दिन्छ।

The analogy that we made is also true for the JavaScript promise object. The constructor syntax for the promise object is:

हामीले बनाएको सादृश्य जाभास्क्रिप्ट 'प्रतिज्ञा' वस्तुको लागि पनि सत्य छ। 'प्रतिज्ञा' वस्तुका लागि कन्स्ट्रक्टर वाक्यविन्यास हो।

let promise = new Promise(function(resolve, reject) {
  // executor (the producing code, "writer")
});

Here, a function is passed to new Promise also known as the executor, and runs automatically upon creation. It contains the producing code that gives the result. resolve and rejects are the arguments provided by the JavaScript itself and are called one of these upon results.

यहाँ, एउटा प्रकार्य 'नयाँ प्रतिज्ञा' मा पास गरिएको छ जसलाई निष्पादक पनि भनिन्छ, र सिर्जनामा स्वचालित रूपमा चल्छ। यसले उत्पादन कोड समावेश गर्दछ जुन परिणाम दिन्छ। 'रिजोल्भ' र 'रिजेक्ट' जाभास्क्रिप्टद्वारा नै प्रदान गरिएका तर्कहरू हुन् र परिणामहरूमा यीमध्ये एक भनिन्छ।

• resolve(value): a callback function that returns value upon result (एक कलब्याक प्रकार्य जसले परिणाममा 'मान' फर्काउँछ)

• reject(error): a callback function that returns error upon error, it returns an error object (कलब्याक प्रकार्य जसले त्रुटिमा 'त्रुटि' फर्काउँछ, यसले त्रुटि वस्तु फर्काउँछ)

The internal properties of promise object returned by the new Promise constructor are as follows:

'नयाँ प्रतिज्ञा' कन्स्ट्रक्टरद्वारा फिर्ता गरिएको 'प्रतिज्ञा' वस्तुको आन्तरिक गुणहरू निम्नानुसार छन्:

• state - initially pending, then changes to either fulfill upon resolve or rejected when reject is called (सुरुमा 'पेन्डिङ', त्यसपछि समाधानमा 'पूरा' वा 'अस्वीकार' गर्दा 'अस्वीकृत' मा परिवर्तन हुन्छ)

• result - initially undefined, then changes to value upon resolve or error when reject is called (सुरुमा 'अपरिभाषित', त्यसपछि 'रिजेक्ट' गर्दा 'रिजोल्युसन' वा 'त्रुटि' मा 'मान' मा परिवर्तन हुन्छ जब 'अस्वीकार' भनिन्छ)

Example of a promise.

प्रतिज्ञा को उदाहरण।

let promiseOne = new Promise(function(resolve, reject) {
  // the function is executed automatically when the promise is constructed

  // after 1-second signal that the job is done with the result "done"
  setTimeout(() => resolve("done"), 1000);
})

let promiseTwo = new Promise(function(resolve, reject) {
  // the function is executed automatically when the promise is constructed

  // after 1-second signal that the job is done with the result "error"
  setTimeout(() => reject(new Error("Whoops!")), 1000);
})

Here, the promiseOne is an example of a "fulfilled promise" as it successfully resolves the values, whereas the promiseTwo is a "rejected promise" as it gets rejected. A promise that is either rejected or resolved is called a settled promise, as opposed to an initially pending promise. Consuming function from the promise can be registered using the .then and .catch methods. We can also add .finally method for performing cleanup or finalizing after previous methods have been completed.

यहाँ, promiseOne एक "fulfilled promise" को एक उदाहरण हो किनकि यसले सफलतापूर्वक मानहरू समाधान गर्दछ, जबकि promiseTwo एक "rejected promise" हो किनकि यो अस्वीकार हुन्छ। अस्वीकृत वा समाधान गरिएको प्रतिज्ञालाई सुरुमा पेन्डिङ गरिएको प्रतिज्ञाको विपरीत सेटल प्रतिज्ञा भनिन्छ। प्रतिज्ञाबाट प्रकार्य उपभोग गर्दा .then र .catch विधिहरू प्रयोग गरेर दर्ता गर्न सकिन्छ। हामी पहिलेका विधिहरू पूरा भएपछि सफाई वा अन्तिम रूप दिनका लागि .finally विधि पनि थप्न सक्छौं।

let promiseOne = new Promise(function(resolve, reject) {
  setTimeout(() => resolve("done!"), 1000);
});

// resolve runs the first function in .then
promiseOne.then(
  result => alert(result), // shows "done!" after 1 second
  error => alert(error) // doesn't run
);

let promiseTwo = new Promise(function(resolve, reject) {
  setTimeout(() => reject(new Error("Whoops!")), 1000);
});

// reject runs the second function in .then
promiseTwo.then(
  result => alert(result), // doesn't run
  error => alert(error) // shows "Error: Whoops!" after 1 second
);

let promiseThree = new Promise((resolve, reject) => {
  setTimeout(() => reject(new Error("Whoops!")), 1000);
});

// .catch(f) is the same as promise.then(null, f)
promiseThree.catch(alert); // shows "Error: Whoops!" after 1 second