我想创建自己的fetch函数,以了解更好的xmlhttprequest,promises和异步/等待。
当我遇到错误时()
,这似乎并没有回报承诺
const fakeFetch = url => {
const xhr = () => new Promise((resolve, reject) => {
try {
const x = new XMLHttpRequest();
x.onreadystatechange = function() {
const { readyState, status } = this;
if (readyState === 4 && status === 200) {
resolve(x.responseText);
}
}
x.open('get', url);
x.send();
} catch(e) {
reject(e);
}
})
const _fetch = () => new Promise(async (resolve, reject) => {
try {
const response = await xhr();
if (response !== undefined) resolve(response);
} catch(e) {
reject(e);
}
})
_fetch();
}
fakeFetch('https://api.github.com/users')
.then(data => data.json())
.then(data => console.log(data));
您将async
关键字放在错误的位置,应该在这里:
let p = new Promise(async (resolve, reject) => {
^^^^^
和 await
的功能,它必须返回承诺:(但是在这种用例中,您实际上不需要await
)
const fakeFetch = url => {
const xhr = () => new Promise((resolve, reject) => {
try {
const x = new XMLHttpRequest();
x.onreadystatechange = function() {
const {
readyState,
status
} = this;
if (readyState === 4 && status === 200) {
resolve(x.responseText);
}
}
x.open('get', url);
x.send();
} catch (e) {
reject(e);
}
})
const _fetch = () => new Promise((resolve, reject) => {
try {
const response = xhr();
if (response !== undefined) resolve(response);
} catch (e) {
reject(e);
}
})
return _fetch();
}
fakeFetch('https://api.github.com/users')
.then(data => console.log(data));