repl: Assignment of _ allowed with warning

This commit addresses https://github.com/nodejs/node/issues/5431 by
changing the way that the repl handles assignment to the global _
variable.

Prior to this commit, node sets the result of the last expression
evaluated in the repl to `_`. This causes problems for users of
underscore, lodash and other packages where it is common to assign
`_` to the package, e.g. `_ = require('lodash');`.

Changes in this commit now result in the following behavior.

- If unassigned on the repl, `_` continues to refer to the last
  evaluated expression.
- If assigned, the default behavior of assigning `_` to the last
  evaluated expression is disabled, and `_` now references whatever
  value was explicitly set. A warning is issued on the repl -
  'expression assignment to _ now disabled'.
- If `_` is assigned multiple times, the warning is only displayed once.
- When `.clear` is executed in the repl, `_` continues to refer to its
  most recent value, whatever that is (this is per existing behavior).
  If `_` had been explicitly set prior to `.clear` it will not change
  again with the evaluation of the next expression.

PR-URL: https://github.com/nodejs/node/pull/5535
Fixes: https://github.com/nodejs/node/issues/5431
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>

1 files changed, 156 insertions, 0 deletions

diff --git a/test/parallel/test-repl-underscore.js b/test/parallel/test-repl-underscore.js
new file mode 100644
index 0000000000..97fc508ad9
--- /dev/null
+++ b/test/parallel/test-repl-underscore.js
@@ -0,0 +1,156 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const repl = require('repl');
+const stream = require('stream');
+
+testSloppyMode();
+testStrictMode();
+testResetContext();
+testMagicMode();
+
+function testSloppyMode() {
+  const r = initRepl(repl.REPL_MODE_SLOPPY);
+
+  // cannot use `let` in sloppy mode
+  r.write(`_;          // initial value undefined
+          var x = 10;  // evaluates to undefined
+          _;           // still undefined
+          y = 10;      // evaluates to 10
+          _;           // 10 from last eval
+          _ = 20;      // explicitly set to 20
+          _;           // 20 from user input
+          _ = 30;      // make sure we can set it twice and no prompt
+          _;           // 30 from user input
+          y = 40;      // make sure eval doesn't change _
+          _;           // remains 30 from user input
+          `);
+
+  assertOutput(r.output, [
+    'undefined',
+    'undefined',
+    'undefined',
+    '10',
+    '10',
+    'Expression assignment to _ now disabled.',
+    '20',
+    '20',
+    '30',
+    '30',
+    '40',
+    '30'
+  ]);
+}
+
+function testStrictMode() {
+  const r = initRepl(repl.REPL_MODE_STRICT);
+
+  r.write(`_;          // initial value undefined
+          var x = 10;  // evaluates to undefined
+          _;           // still undefined
+          let _ = 20;  // use 'let' only in strict mode - evals to undefined
+          _;           // 20 from user input
+          _ = 30;      // make sure we can set it twice and no prompt
+          _;           // 30 from user input
+          var y = 40;  // make sure eval doesn't change _
+          _;           // remains 30 from user input
+          function f() { let _ = 50; } // undefined
+          f();         // undefined
+          _;           // remains 30 from user input
+          `);
+
+  assertOutput(r.output, [
+    'undefined',
+    'undefined',
+    'undefined',
+    'undefined',
+    '20',
+    '30',
+    '30',
+    'undefined',
+    '30',
+    'undefined',
+    'undefined',
+    '30'
+  ]);
+}
+
+function testMagicMode() {
+  const r = initRepl(repl.REPL_MODE_MAGIC);
+
+  r.write(`_;          // initial value undefined
+          x = 10;      //
+          _;           // last eval - 10
+          let _ = 20;  // undefined
+          _;           // 20 from user input
+          _ = 30;      // make sure we can set it twice and no prompt
+          _;           // 30 from user input
+          var y = 40;  // make sure eval doesn't change _
+          _;           // remains 30 from user input
+          function f() { let _ = 50; return _; } // undefined
+          f();         // 50
+          _;           // remains 30 from user input
+          `);
+
+  assertOutput(r.output, [
+    'undefined',
+    '10',
+    '10',
+    'undefined',
+    '20',
+    '30',
+    '30',
+    'undefined',
+    '30',
+    'undefined',
+    '50',
+    '30'
+  ]);
+}
+
+function testResetContext() {
+  const r = initRepl(repl.REPL_MODE_SLOPPY);
+
+  r.write(`_ = 10;     // explicitly set to 10
+          _;           // 10 from user input
+          .clear       // Clearing context...
+          _;           // remains 10
+          x = 20;      // but behavior reverts to last eval
+          _;           // expect 20
+          `);
+
+  assertOutput(r.output, [
+    'Expression assignment to _ now disabled.',
+    '10',
+    '10',
+    'Clearing context...',
+    '10',
+    '20',
+    '20'
+  ]);
+}
+
+function initRepl(mode) {
+  const inputStream = new stream.PassThrough();
+  const outputStream = new stream.PassThrough();
+  outputStream.accum = '';
+
+  outputStream.on('data', (data) => {
+    outputStream.accum += data;
+  });
+
+  return repl.start({
+    input: inputStream,
+    output: outputStream,
+    useColors: false,
+    terminal: false,
+    prompt: '',
+    replMode: mode
+  });
+}
+
+function assertOutput(output, expected) {
+  const lines = output.accum.trim().split('\n');
+  assert.deepStrictEqual(lines, expected);
+}