From 16b45dbaa2e6eefb979127be4c4464675cb43e71 Mon Sep 17 00:00:00 2001
From: rmdm <rmdm@users.noreply.github.com>
Date: Tue, 5 Mar 2019 21:07:33 +0300
Subject: [PATCH] fill not passed args with undefined and pass cb as the last
 one

---
 package.json          |  2 +-
 promback.js           | 17 ++++++++++++-----
 test/promback.spec.js | 14 ++++++++++++++
 3 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/package.json b/package.json
index 81d2fbe..86cbe9e 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "promback",
-  "version": "1.0.0",
+  "version": "1.0.1",
   "description": "Level promises/callbacks landscape",
   "main": "promback.js",
   "scripts": {
diff --git a/promback.js b/promback.js
index 6354f60..bc278c2 100644
--- a/promback.js
+++ b/promback.js
@@ -16,14 +16,21 @@ function using (PromiseLib) {
 
             return new PromiseLib(function (resolve, reject) {
 
-                if (fn.length !== args.length + 1) {
+                const argsDiff = fn.length - args.length
+
+                if (argsDiff <= 0) {
                     return resolve(fn.apply(that, args))
                 }
 
-                fn.call(that, ... args, function (err, value) {
-                    if (err) { return reject(err) }
-                    resolve(value)
-                })
+                fn.call(
+                    that,
+                    ... args,
+                    ... new Array(argsDiff - 1),
+                    function (err, value) {
+                        if (err) { return reject(err) }
+                        resolve(value)
+                    }
+                )
             })
         }
     }
diff --git a/test/promback.spec.js b/test/promback.spec.js
index 89e49bc..42c11a0 100644
--- a/test/promback.spec.js
+++ b/test/promback.spec.js
@@ -148,6 +148,20 @@ function testUsing (contextTitle, promiseLib) {
 
                 assert.strictEqual(result, undefined)
             })
+
+            it('passes cb as the last arg, when the function expects more args than passed',
+                async function () {
+
+                function testFn (a, b, c, cb) {
+                    cb(null, [ a, b, c ])
+                }
+
+                const fn = promback(testFn)
+
+                const result = await fn(1)
+
+                assert.deepStrictEqual(result, [ 1, undefined, undefined ])
+            })
         })
     })
 }