Why fetch returns promise pending?(为什么 fetch 返回 promise 未决?)

问题描述

I am using fetch to get data but it keeps returning promise as pending. I've seen many posts regarding this issue and tried all the possibilities but didn't solve my issue. I wanted to know why the fetch returns promise as pending in brief what are the possible cases where fetch returns pending status?

My piece of code for reference:

fetch(res.url).then(function(u){ 
    return u.json();
})
.then(function(j) { 
    console.log(j); 
});

Promises are a way to allow callers do other work while waiting for result of the function.

See Promises and Using Promises on MDN:

A Promise is in one of these states:

• pending: initial state, neither fulfilled nor rejected.
• fulfilled: meaning that the operation completed successfully.
• rejected: meaning that the operation failed.

The fetch(url) returns a Promise object. It allows attaching "listener" to it using .then(…) that can respond to result value (response to the request). The .then(…) returns again Promise object that will give result forward.

async and await

You can use JS syntax sugar for using Promises:

async function my_async_fn(url) {
    let response = await fetch(url);
    console.log(response); // Logs the response
    return response;
)

console.log(my_async_fn(url)); // Returns Promise

async functions return a Promise. await keyword wraps rest of the function in .then(…). Here is equivalent without await and async:

// This function also returns Promise
function my_async_fn(url) {
    return fetch(url).then(response => {
        console.log(response); // Logs the response
        return response;
    });
)

console.log(my_async_fn(url)); // Returns Promise

Again see article on Promises on MDN.

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