doc: clarify usage of util.promisify.custom

PR-URL: #16134 Reviewed-By: Anna Henningsen <[email protected]> Reviewed-By: Stephen Belanger <[email protected]> Reviewed-By: Jeremiah Senkpiel <[email protected]> Reviewed-By: Evan Lucas <[email protected]> Reviewed-By: James M Snell <[email protected]>
nodejs · Oct 18, 2017 · 55fa067 · 55fa067
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commit 55fa067
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diff --git a/doc/api/util.md b/doc/api/util.md
@@ -525,6 +525,16 @@ console.log(promisified === doSomething[util.promisify.custom]);
 This can be useful for cases where the original function does not follow the
 standard format of taking an error-first callback as the last argument.
 
+For example, with a function that takes in `(foo, onSuccessCallback, onErrorCallback)`:
+
+```js
+doSomething[util.promisify.custom] = function(foo) {
+  return new Promise(function(resolve, reject) {
+    doSomething(foo, resolve, reject);
+  });
+};
+```
+
 ### util.promisify.custom
 <!-- YAML
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