update

tutorials/1_promises.js

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+// https://javascript.info/promise-basics
+
+/*
+    The function passed to Promise is called "executor". When Promise gets
+    created, the executor gets executed.
+    When the Promise ends, it should either call the "resolve" or "reject"
+    callbacks:
+    resolve(value) — if the job is finished successfully, with result value.
+    reject(error) — if an error has occurred, error is the error object.
+*/
+let promise = new Promise(function(resolve, reject) {
+    setTimeout(() => resolve("done"), 500);
+  });
+
+/*
+    The first argument of .then is a function that runs when the promise is resolved and receives the result.
+    The second argument of .then is a function that runs when the promise is rejected and receives the error.
+*/
+promise.then(
+    result => console.log("The operation was successful. It returned " + result),
+    error => console.log("The operation was not successful: " + error)
+);
+
+/*
+    Or we can pass only one argument if we're interested only in a positive result
+*/
+promise.then(
+    result => console.log("The operation was successful. It returned " + result)
+);
+
+/*
+    Or we can pass only one argument to the method "catch" if we're interested
+    in negative results only.
+
+    promise.catch internally just calls promise.then(null, f)
+*/
+promise.catch(
+    error => console.log(error)
+);
+
+/*
+    finally gets always called
+*/
+promise.finally(
+    () => console.log("The execution has terminated. Bye")
+);