readline: allow history to be disabled

fix #6336
1.  The `historySize` to default to `30` only if `undefined`.
2.  If `historySize` is set to 0, then disable caching the line.
3.  Added unit tests.
4.  Updated documentation.

doc/api/readline.md

@@ -300,7 +300,8 @@ the following values:
    treated like a TTY, and have ANSI/VT100 escape codes written to it.
    Defaults to checking `isTTY` on the `output` stream upon instantiation.
 
- - `historySize` - maximum number of history lines retained. Defaults to `30`.
+ - `historySize` - maximum number of history lines retained. To disable the
+   history set this value to `0`. Defaults to `30`.
 
 The `completer` function is given the current line entered by the user, and
 is supposed to return an Array with 2 entries:

lib/readline.js

@@ -53,12 +53,15 @@ function Interface(input, output, completer, terminal) {
     historySize = input.historySize;
     input = input.input;
   }
-  historySize = historySize || kHistorySize;
 
   if (completer && typeof completer !== 'function') {
     throw new TypeError('Argument "completer" must be a function');
   }
 
+  if (historySize === undefined) {
+    historySize = kHistorySize;
+  }
+
   if (typeof historySize !== 'number' ||
       isNaN(historySize) ||
       historySize < 0) {
@@ -228,6 +231,9 @@ Interface.prototype._writeToOutput = function _writeToOutput(stringToWrite) {
 Interface.prototype._addHistory = function() {
   if (this.line.length === 0) return '';
 
+  // if the history is disabled then return the line
+  if (this.historySize === 0) return this.line;
+
   if (this.history.length === 0 || this.history[0] !== this.line) {
     this.history.unshift(this.line);
 

test/parallel/test-readline-interface.js

@@ -27,6 +27,19 @@ function isWarned(emitter) {
   var rli;
   var called;
 
+  // disable history
+  fi = new FakeInput();
+  rli = new readline.Interface({ input: fi, output: fi, terminal: terminal,
+                              historySize: 0 });
+  assert.strictEqual(rli.historySize, 0);
+  rli.close();
+
+  // default history size 30
+  fi = new FakeInput();
+  rli = new readline.Interface({ input: fi, output: fi, terminal: terminal});
+  assert.strictEqual(rli.historySize, 30);
+  rli.close();
+
   // sending a full line
   fi = new FakeInput();
   rli = new readline.Interface({ input: fi, output: fi, terminal: terminal });