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repl: Assignment of _ allowed with warning

This commit addresses #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: #5535
Fixes: #5431
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>
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lance authored and silverwind committed Mar 2, 2016
1 parent c67937b commit ad8257fa5b9795d3d79fa4a91d0f18c43f024ab3
Showing with 183 additions and 5 deletions.
  1. +2 −0 doc/api/repl.markdown
  2. +25 −5 lib/repl.js
  3. +156 −0 test/parallel/test-repl-underscore.js
@@ -86,6 +86,8 @@ The special variable `_` (underscore) contains the result of the last expression
4
```

Explicitly setting `_` will disable this behavior until the context is reset.

The REPL provides access to any variables in the global scope. You can expose
a variable to the REPL explicitly by assigning it to the `context` object
associated with each `REPLServer`. For example:
@@ -205,6 +205,8 @@ function REPLServer(prompt,
self.useGlobal = !!useGlobal;
self.ignoreUndefined = !!ignoreUndefined;
self.replMode = replMode || exports.REPL_MODE_SLOPPY;
self.underscoreAssigned = false;
self.last = undefined;

self._inTemplateLiteral = false;

@@ -471,7 +473,9 @@ function REPLServer(prompt,
// the second argument to this function is there, print it.
arguments.length === 2 &&
(!self.ignoreUndefined || ret !== undefined)) {
self.context._ = ret;
if (!self.underscoreAssigned) {
self.last = ret;
}
self.outputStream.write(self.writer(ret) + '\n');
}

@@ -545,27 +549,43 @@ REPLServer.prototype.createContext = function() {
context.module = module;
context.require = require;


this.underscoreAssigned = false;
this.lines = [];
this.lines.level = [];

// make built-in modules available directly
// (loaded lazily)
exports._builtinLibs.forEach(function(name) {
exports._builtinLibs.forEach((name) => {
Object.defineProperty(context, name, {
get: function() {
get: () => {
var lib = require(name);
context._ = context[name] = lib;
this.last = context[name] = lib;
return lib;
},
// allow the creation of other globals with this name
set: function(val) {
set: (val) => {
delete context[name];
context[name] = val;
},
configurable: true
});
});

Object.defineProperty(context, '_', {
configurable: true,
get: () => {
return this.last;
},
set: (value) => {
this.last = value;
if (!this.underscoreAssigned) {
this.underscoreAssigned = true;
this.outputStream.write('Expression assignment to _ now disabled.\n');
}
}
});

return context;
};

@@ -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);
}

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