62 lines
2.1 KiB
JavaScript
62 lines
2.1 KiB
JavaScript
// https://javascript.info/promise-basics
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/*
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The function passed to Promise is called "executor"
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and gets executed synchronously and immediately after the Promise is created.
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When the Promise ends, the callback function should
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either call the "resolve" or "reject" callbacks:
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resolve(value) — if the job is finished successfully, with result value.
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reject(error) — if an error has occurred, error is the error object.
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A Promise can be in one of these three states:
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- Pending (the operation is being processed)
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- Fullfilled (the operation has completed successfully, resolve has been called)
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- Rejected (the operation has not completed successfully, reject has been called)
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A promise represents the completion of a (likely) asynchronous function. It is an object
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that might return a value in the future. It accomplishes the same basic goal as a
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callback function, but with many additional features and a more readable syntax.
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*/
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const promise = new Promise(function (resolve, reject) {
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setTimeout(() => resolve('done'), 5000)
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})
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/*
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The first argument of .then is a function that runs when the promise is resolved and receives the result.
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The second argument of .then is a function that runs when the promise is rejected and receives the error.
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*/
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promise.then(
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result => console.log('The operation was successful. It returned ' + result),
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error => console.log('The operation was not successful: ' + error)
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)
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/*
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Or we can pass only one argument if we're interested only in a positive result
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*/
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promise.then(
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result => console.log('The operation was successful. It returned ' + result)
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)
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/*
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Or we can pass only one argument to the method "catch" if we're interested
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in negative results only.
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promise.catch internally just calls promise.then(null, f)
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*/
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promise.catch(
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error => console.log(error)
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)
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/*
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finally gets always called
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*/
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promise.finally(
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() => console.log('The execution has terminated. Bye')
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)
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/*
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This line is used to demonstrate that the code within "then, catch, etc."
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is in the event loop, as this is the first line getting executed.
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*/
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console.log('Last line')
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