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September 30, 2019 18:34
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Lazy promises
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| Promise.cancelable = pendingPromise => { | |
| let lazyCb = null; | |
| let settled = false; | |
| let canceled = false; | |
| const promise = Promise.resolve(pendingPromise).then(() => { | |
| settled = true; | |
| if (canceled) { | |
| return; | |
| } | |
| return lazyCb && pendingPromise.then(lazyCb); | |
| }); | |
| return { | |
| then: cb => { | |
| if (canceled) { | |
| return; | |
| } | |
| if (settled) { | |
| promise.then(cb); | |
| return; | |
| } | |
| lazyCb = cb; | |
| }, | |
| cancel: () => { | |
| if (!settled) { | |
| canceled = true; | |
| } | |
| }, | |
| }; | |
| }; |
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