feat(dns): added support for a custom lookup function; (#5339)

lib/utils.js

@@ -662,6 +662,15 @@ const toJSONObject = (obj) => {
   return visit(obj, 0);
 }
 
+const [isPlainFunction, isAsyncFn, isGeneratorFn, isAsyncGeneratorFn] = (
+  (...fns) => fns.map(
+    ({constructor})=> (thing) => thing && typeof thing === 'function' && thing.constructor === constructor
+  )
+)(()=> {}, async()=>{}, function*(){}, async function*(){});
+
+const isThenable = (thing) =>
+  thing && (isObject(thing) || isFunction(thing)) && isFunction(thing.then) && isFunction(thing.catch);
+
 export default {
   isArray,
   isArrayBuffer,
@@ -711,5 +720,10 @@ export default {
   ALPHABET,
   generateString,
   isSpecCompliantForm,
-  toJSONObject
+  toJSONObject,
+  isAsyncFn,
+  isGeneratorFn,
+  isAsyncGeneratorFn,
+  isPlainFunction,
+  isThenable
 };