取消 JavaScript 承诺

Question

我正在尝试取消以下承诺：

function example(cancel = Promise.reject()) {
return new Promise((resolve, reject) => {
const timer = setTimeout(() => resolve('jack-jack'), 5000);
cancel.then((res) => {
clearTimeout(timer);
reject('cancelled'); 
}, ()=>{})
});
}
var cancel=Promise.reject(); 
example(cancel).then((res) => console.log('res handled:' + res)).catch((err) => console.log('err handled:' + err));
console.log('attempting cancellation of promise');
cancel=Promise.resolve();

但是我无法取消它。我在这里做错了什么？

Answer 1

在你的代码中，你已经向函数传递了一个完整的(被拒绝的(承诺。cancel=Promise.resolve();之后attempting cancellation of promise不会对传递给example的承诺产生任何影响，因为您只是创建了一个新的已解决的承诺。

如果要取消正在运行的进程，则可能需要选择以下解决方案：

function example(helper) {
return new Promise((resolve, reject) => {
helper.cancel = function() {
clearTimeout(timer)
reject('cancelled');
}
const timer = setTimeout(() => resolve('jack-jack'), 5000);
});
}
var helper = {};
example(helper).then((res) => console.log('res handled:' + res)).catch((err) => console.log('err handled:' + err));
console.log('attempting cancellation of promise');
helper.cancel()

Answer 2

您已将被拒绝的承诺分配给变量，将被拒绝的承诺传递到函数中，并将已解析的承诺分配给变量。变量采用的两个值是不相关的，您无法更改已结算承诺的状态。

传递您可以解决的承诺：

let cancel;
let cancelPromise = new Promise((resolve) => {
cancel = resolve;
});
example(cancelPromise).then(…).catch(…);
console.log('attempting cancellation of promise');
cancel();

function example(cancel = Promise.reject()) {
return new Promise((resolve, reject) => {
const timer = setTimeout(() => resolve('jack-jack'), 5000);
cancel.then((res) => {
clearTimeout(timer);
reject('cancelled'); 
}, ()=>{})
});
}
let cancel;
let cancelPromise = new Promise((resolve) => {
cancel = resolve;
});
example(cancelPromise)
.then((res) => console.log('res handled:' + res))
.catch((err) => console.log('err handled:' + err));
console.log('attempting cancellation of promise');
cancel();

Answer 3

由于取消设置为拒绝，因此

cancel.then((res) => { //you cannot use then for reject, as reject cannot be resolved.
clearTimeout(timer);
reject('cancelled'); 
}, ()=>{})

永远不会被执行，因此你必须解决它，而不是拒绝它

function example(cancel = Promise.reject()) {
return new Promise((resolve, reject) => {
const timer = setTimeout(() => resolve('jack-jack'), 5000);
cancel.then((res) => { //you cannot use then for reject
clearTimeout(timer);
reject('cancelled'); 
}, ()=>{})
});
}
var cancel = Promise.resolve(); // just change to resolve
example(cancel).then((res) => console.log('res handled:' + res)).catch((err) => console.log('err handled:' + err));
console.log('attempting cancellation of promise');
cancel=Promise.resolve(); // this will have no effect as you have already passed a reject

Answer 4

因为你传递了被拒绝的承诺。如果您希望块运行cancel.then()请传递已解决的承诺。

function example(cancel = Promise.resolve()) {
return new Promise((resolve, reject) => {
console.log(cancel);
const timer = setTimeout(() => {
resolve('jack-jack'), 5000
});
cancel.then((res) => {
console.log('CANCELLED');
clearTimeout(timer);
reject('cancelled');
}, () => {})
});
}
var cancel = Promise.resolve();
example(cancel).then((res) => console.log('res handled:' + res)).catch((err) => console.log('err handled:' + err));
console.log('attempting cancellation of promise');
cancel = Promise.resolve();