diff --git a/lib/readline.js b/lib/readline.js
index e7c5a04bb03eed..e847ce961f53ad 100644
--- a/lib/readline.js
+++ b/lib/readline.js
@@ -94,8 +94,8 @@ function Interface(input, output, completer, terminal) {
   this.escapeCodeTimeout = ESCAPE_CODE_TIMEOUT;
 
   EventEmitter.call(this);
-  var historySize;
-  var removeHistoryDuplicates = false;
+  let historySize;
+  let removeHistoryDuplicates = false;
   let crlfDelay;
   let prompt = '> ';
 
@@ -258,10 +258,9 @@ Object.defineProperty(Interface.prototype, 'columns', {
   configurable: true,
   enumerable: true,
   get: function() {
-    var columns = Infinity;
     if (this.output && this.output.columns)
-      columns = this.output.columns;
-    return columns;
+      return this.output.columns;
+    return Infinity;
   }
 });
 
@@ -308,7 +307,7 @@ Interface.prototype.question = function(query, cb) {
 
 Interface.prototype._onLine = function(line) {
   if (this._questionCallback) {
-    var cb = this._questionCallback;
+    const cb = this._questionCallback;
     this._questionCallback = null;
     this.setPrompt(this._oldPrompt);
     cb(line);
@@ -435,7 +434,7 @@ Interface.prototype._normalWrite = function(b) {
   if (b === undefined) {
     return;
   }
-  var string = this._decoder.write(b);
+  let string = this._decoder.write(b);
   if (this._sawReturnAt &&
       Date.now() - this._sawReturnAt <= this.crlfDelay) {
     string = string.replace(/^\n/, '');
@@ -453,11 +452,11 @@ Interface.prototype._normalWrite = function(b) {
     this._sawReturnAt = string.endsWith('\r') ? Date.now() : 0;
 
     // Got one or more newlines; process into "line" events
-    var lines = string.split(lineEnding);
+    const lines = string.split(lineEnding);
     // Either '' or (conceivably) the unfinished portion of the next line
     string = lines.pop();
     this._line_buffer = string;
-    for (var n = 0; n < lines.length; n++)
+    for (let n = 0; n < lines.length; n++)
       this._onLine(lines[n]);
   } else if (string) {
     // No newlines this time, save what we have for next time
@@ -467,8 +466,8 @@ Interface.prototype._normalWrite = function(b) {
 
 Interface.prototype._insertString = function(c) {
   if (this.cursor < this.line.length) {
-    var beg = this.line.slice(0, this.cursor);
-    var end = this.line.slice(this.cursor, this.line.length);
+    const beg = this.line.slice(0, this.cursor);
+    const end = this.line.slice(this.cursor, this.line.length);
     this.line = beg + c + end;
     this.cursor += c.length;
     this._refreshLine();
@@ -505,16 +504,16 @@ Interface.prototype._tabComplete = function(lastKeypressWasTab) {
       // Apply/show completions.
       if (lastKeypressWasTab) {
         self._writeToOutput('\r\n');
-        var width = completions.reduce(function completionReducer(a, b) {
+        const width = completions.reduce(function completionReducer(a, b) {
           return a.length > b.length ? a : b;
         }).length + 2;  // 2 space padding
-        var maxColumns = Math.floor(self.columns / width);
+        let maxColumns = Math.floor(self.columns / width);
         if (!maxColumns || maxColumns === Infinity) {
           maxColumns = 1;
         }
-        var group = [];
-        for (var i = 0; i < completions.length; i++) {
-          var c = completions[i];
+        let group = [];
+        for (let i = 0; i < completions.length; i++) {
+          const c = completions[i];
           if (c === '') {
             handleGroup(self, group, width, maxColumns);
             group = [];
@@ -526,8 +525,8 @@ Interface.prototype._tabComplete = function(lastKeypressWasTab) {
       }
 
       // If there is a common prefix to all matches, then apply that portion.
-      var f = completions.filter((e) => e);
-      var prefix = commonPrefix(f);
+      const f = completions.filter((e) => e);
+      const prefix = commonPrefix(f);
       if (prefix.length > completeOn.length) {
         self._insertString(prefix.slice(completeOn.length));
       }
@@ -543,16 +542,16 @@ function handleGroup(self, group, width, maxColumns) {
     return;
   }
   const minRows = Math.ceil(group.length / maxColumns);
-  for (var row = 0; row < minRows; row++) {
-    for (var col = 0; col < maxColumns; col++) {
-      var idx = row * maxColumns + col;
+  for (let row = 0; row < minRows; row++) {
+    for (let col = 0; col < maxColumns; col++) {
+      const idx = row * maxColumns + col;
       if (idx >= group.length) {
         break;
       }
-      var item = group[idx];
+      const item = group[idx];
       self._writeToOutput(item);
       if (col < maxColumns - 1) {
-        for (var s = 0; s < width - item.length; s++) {
+        for (let s = 0; s < width - item.length; s++) {
           self._writeToOutput(' ');
         }
       }
@@ -570,7 +569,7 @@ function commonPrefix(strings) {
   const sorted = strings.slice().sort();
   const min = sorted[0];
   const max = sorted[sorted.length - 1];
-  for (var i = 0, len = min.length; i < len; i++) {
+  for (let i = 0, len = min.length; i < len; i++) {
     if (min[i] !== max[i]) {
       return min.slice(0, i);
     }
@@ -583,9 +582,9 @@ Interface.prototype._wordLeft = function() {
   if (this.cursor > 0) {
     // Reverse the string and match a word near beginning
     // to avoid quadratic time complexity
-    var leading = this.line.slice(0, this.cursor);
-    var reversed = leading.split('').reverse().join('');
-    var match = reversed.match(/^\s*(?:[^\w\s]+|\w+)?/);
+    const leading = this.line.slice(0, this.cursor);
+    const reversed = leading.split('').reverse().join('');
+    const match = reversed.match(/^\s*(?:[^\w\s]+|\w+)?/);
     this._moveCursor(-match[0].length);
   }
 };
@@ -593,8 +592,8 @@ Interface.prototype._wordLeft = function() {
 
 Interface.prototype._wordRight = function() {
   if (this.cursor < this.line.length) {
-    var trailing = this.line.slice(this.cursor);
-    var match = trailing.match(/^(?:\s+|[^\w\s]+|\w+)\s*/);
+    const trailing = this.line.slice(this.cursor);
+    const match = trailing.match(/^(?:\s+|[^\w\s]+|\w+)\s*/);
     this._moveCursor(match[0].length);
   }
 };
@@ -643,9 +642,9 @@ Interface.prototype._deleteWordLeft = function() {
   if (this.cursor > 0) {
     // Reverse the string and match a word near beginning
     // to avoid quadratic time complexity
-    var leading = this.line.slice(0, this.cursor);
-    var reversed = leading.split('').reverse().join('');
-    var match = reversed.match(/^\s*(?:[^\w\s]+|\w+)?/);
+    let leading = this.line.slice(0, this.cursor);
+    const reversed = leading.split('').reverse().join('');
+    const match = reversed.match(/^\s*(?:[^\w\s]+|\w+)?/);
     leading = leading.slice(0, leading.length - match[0].length);
     this.line = leading + this.line.slice(this.cursor, this.line.length);
     this.cursor = leading.length;
@@ -656,8 +655,8 @@ Interface.prototype._deleteWordLeft = function() {
 
 Interface.prototype._deleteWordRight = function() {
   if (this.cursor < this.line.length) {
-    var trailing = this.line.slice(this.cursor);
-    var match = trailing.match(/^(?:\s+|\W+|\w+)\s*/);
+    const trailing = this.line.slice(this.cursor);
+    const match = trailing.match(/^(?:\s+|\W+|\w+)\s*/);
     this.line = this.line.slice(0, this.cursor) +
                 trailing.slice(match[0].length);
     this._refreshLine();
@@ -723,13 +722,12 @@ Interface.prototype._historyPrev = function() {
 
 // Returns the last character's display position of the given string
 Interface.prototype._getDisplayPos = function(str) {
-  var offset = 0;
+  let offset = 0;
   const col = this.columns;
-  var row = 0;
-  var code;
+  let row = 0;
   str = stripVTControlCharacters(str);
-  for (var i = 0, len = str.length; i < len; i++) {
-    code = str.codePointAt(i);
+  for (let i = 0, len = str.length; i < len; i++) {
+    const code = str.codePointAt(i);
     if (code >= kUTF16SurrogateThreshold) { // Surrogates.
       i++;
     }
@@ -761,8 +759,8 @@ Interface.prototype._getCursorPos = function() {
   const strBeforeCursor = this._prompt + this.line.substring(0, this.cursor);
   const dispPos = this._getDisplayPos(
     stripVTControlCharacters(strBeforeCursor));
-  var cols = dispPos.cols;
-  var rows = dispPos.rows;
+  let cols = dispPos.cols;
+  let rows = dispPos.rows;
   // If the cursor is on a full-width character which steps over the line,
   // move the cursor to the beginning of the next line.
   if (cols + 1 === columns &&
@@ -790,8 +788,8 @@ Interface.prototype._moveCursor = function(dx) {
 
   // Check if cursors are in the same line
   if (oldPos.rows === newPos.rows) {
-    var diffCursor = this.cursor - oldcursor;
-    var diffWidth;
+    const diffCursor = this.cursor - oldcursor;
+    let diffWidth;
     if (diffCursor < 0) {
       diffWidth = -getStringWidth(
         this.line.substring(this.cursor, oldcursor)
@@ -1072,8 +1070,8 @@ Interface.prototype._ttyWrite = function(s, key) {
 
       default:
         if (typeof s === 'string' && s) {
-          var lines = s.split(/\r\n|\n|\r/);
-          for (var i = 0, len = lines.length; i < len; i++) {
+          const lines = s.split(/\r\n|\n|\r/);
+          for (let i = 0, len = lines.length; i < len; i++) {
             if (i > 0) {
               this._line();
             }
@@ -1136,7 +1134,7 @@ function emitKeypressEvents(stream, iface) {
 
   function onData(b) {
     if (stream.listenerCount('keypress') > 0) {
-      var r = stream[KEYPRESS_DECODER].write(b);
+      const r = stream[KEYPRESS_DECODER].write(b);
       if (r) {
         clearTimeout(timeoutId);
 
@@ -1144,7 +1142,7 @@ function emitKeypressEvents(stream, iface) {
           iface._sawKeyPress = r.length === 1;
         }
 
-        for (var i = 0; i < r.length; i++) {
+        for (let i = 0; i < r.length; i++) {
           if (r[i] === '\t' && typeof r[i + 1] === 'string' && iface) {
             iface.isCompletionEnabled = false;
           }