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--- lwc:programming:javascript [2022/11/15 08:35] – John Harrison
+++ lwc:programming:javascript [2025/03/18 13:58] (current) – John Harrison
@@ Line 52: / Line 52: @@
   * to see all the event messages flying around: ''monitorEvents(window);''
   * then to unsee: ''unmonitorEvents(window);''
+==== Promises ====
+Promises are always async and often used to prevent UI blocking when waiting for something like an I/O interface. If a function has a ''.then'' then it is definitely calling a promise (or a "thenable" object but yeah that doesn't happen)
+=== Creating the promise (producing) ===
+<code>
+myPromise = Promise(success, failure) {
+  // do something that takes some time here
+  if (succeeded) {
+    success() { // what to return on success i.e. "3" or "hello"}; // this is not a function. It's just a return value
+  else
+    failure() { // what to return on failure }; // this is not a function but only a return value
+}
+</code>
+=== Calling or running the promise (consuming) ===
+== Using .then(onFulfilled, onRejected) ==
+//this is the less preferred method//
+<code>
+myPromise.then(
+  successFunction(value) { // do stuff here with value which was returned by 'success' above }
+  failFunction(value) { // do stuff here with value returned by 'failure' above })
+</code>
+== Using .catch ==
+// this is best practice in general //
+<code>
+myPromise.then(
+  successFunction(value) { // do stuff here with value returned by 'success' when declaring promise })
+  .catch(failFunction(value) { // do stuff here with value returned by 'failure' above. also called if an error in successFunction })
+</code>
+==== Arrow Notation ====
+^ Category ^ Old Skool       ^ Arrow Function          ^
+| No Parameter    | function fred() = {}     | () => {}       |
+| One Parameter   | function fred(a) = {} | (a) => {} or a => {} (parens are optional when only 1 argument)|
+| Two Parameters  | function fred(a,b) = {}     | (a,b) => {}        |