Cancellable Promise Example

cpromise.js

function CPromise (fn) {
    let obj = {}
    let p = new Promise((resolve, reject) => {
        fn(resolve, reject, obj)
        obj.reject = reject
    })
    p.cancel = () => {
        if (obj.oncancel) {
            obj.oncancel()
        }
        obj.reject('cancelled')
    }
    return p
}

var p = CPromise((resolve, reject, p) => {
    let timeout = setTimeout(() => {
        console.log('is completed!!')
        resolve('completed')
    }, 2000)
    p.oncancel = () => clearTimeout(timeout)
})

p.then(r => console.log(r)).catch(err => console.log('Error: ' + err))

setTimeout(() => p.cancel(), 1000)