diff --git a/lib/cli/collect-files.js b/lib/cli/collect-files.js
index 9ea7e83fd2..c7b84b4945 100644
--- a/lib/cli/collect-files.js
+++ b/lib/cli/collect-files.js
@@ -6,6 +6,7 @@ const debug = require('debug')('mocha:cli:run:helpers');
 const minimatch = require('minimatch');
 const {NO_FILES_MATCH_PATTERN} = require('../errors').constants;
 const lookupFiles = require('./lookup-files');
+const {castArray} = require('../utils');
 
 /**
  * Exports a function that collects test files from CLI parameters.
@@ -21,45 +22,44 @@ const lookupFiles = require('./lookup-files');
  * @returns {string[]} List of files to test
  * @private
  */
-module.exports = ({ignore, extension, file, recursive, sort, spec} = {}) => {
-  let files = [];
+module.exports = ({
+  ignore,
+  extension,
+  file: fileArgs,
+  recursive,
+  sort,
+  spec
+} = {}) => {
   const unmatched = [];
-  spec.forEach(arg => {
-    let newFiles;
+  const specFiles = spec.reduce((specFiles, arg) => {
     try {
-      newFiles = lookupFiles(arg, extension, recursive);
+      const moreSpecFiles = castArray(lookupFiles(arg, extension, recursive))
+        .filter(filename =>
+          ignore.every(pattern => !minimatch(filename, pattern))
+        )
+        .map(filename => path.resolve(filename));
+      return [...specFiles, ...moreSpecFiles];
     } catch (err) {
       if (err.code === NO_FILES_MATCH_PATTERN) {
         unmatched.push({message: err.message, pattern: err.pattern});
-        return;
+        return specFiles;
       }
 
       throw err;
     }
-
-    if (typeof newFiles !== 'undefined') {
-      if (typeof newFiles === 'string') {
-        newFiles = [newFiles];
-      }
-      newFiles = newFiles.filter(fileName =>
-        ignore.every(pattern => !minimatch(fileName, pattern))
-      );
-    }
-
-    files = files.concat(newFiles);
-  });
-
-  const fileArgs = file.map(filepath => path.resolve(filepath));
-  files = files.map(filepath => path.resolve(filepath));
+  }, []);
 
   // ensure we don't sort the stuff from fileArgs; order is important!
   if (sort) {
-    files.sort();
+    specFiles.sort();
   }
 
   // add files given through --file to be ran first
-  files = fileArgs.concat(files);
-  debug('files (in order): ', files);
+  const files = [
+    ...fileArgs.map(filepath => path.resolve(filepath)),
+    ...specFiles
+  ];
+  debug('test files (in order): ', files);
 
   if (!files.length) {
     // give full message details when only 1 file is missing
@@ -70,7 +70,7 @@ module.exports = ({ignore, extension, file, recursive, sort, spec} = {}) => {
     console.error(ansi.red(noneFoundMsg));
     process.exit(1);
   } else {
-    // print messages as an warning
+    // print messages as anwarning
     unmatched.forEach(warning => {
       console.warn(ansi.yellow(`Warning: ${warning.message}`));
     });