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copied!<p>Updated note: <a href="https://bugs.chromium.org/p/chromium/issues/detail?id=499864" rel="nofollow noreferrer">this has been fixed in Chrome 49</a>.</p> <p>Very interesting question! Let's dig in.</p> <h2>The root cause</h2> <p>The root of the difference is in how Node.js evaluates these statements vs. how the Chrome development tools do.</p> <h2>What Node.js does</h2> <p>Node.js uses the <a href="http://nodejs.org/api/repl.html" rel="nofollow noreferrer">repl</a> module for this.</p> <p>From the Node.js <a href="https://github.com/joyent/node/blob/master/lib/repl.js#L253" rel="nofollow noreferrer">REPL source code</a>:</p> <pre><code>self.eval( '(' + evalCmd + ')', self.context, 'repl', function (e, ret) { if (e && !isSyntaxError(e)) return finish(e); if (typeof ret === 'function' && /^[\r\n\s]*function/.test(evalCmd) || e) { // Now as statement without parens. self.eval(evalCmd, self.context, 'repl', finish); } else { finish(null, ret); } } ); </code></pre> <p>This acts just like running <code>({}+{})</code> in the Chrome developer tools, which also produces <code>"[object Object][object Object]"</code> as you'd expect.</p> <h2>What the chrome developer tools do</h2> <p>On the other hand <a href="https://chromium.googlesource.com/chromium/blink.git/+/master/Source/core/inspector/InjectedScriptSource.js" rel="nofollow noreferrer">Chrome dveloper tools does the following</a>:</p> <pre><code>try { if (injectCommandLineAPI && inspectedWindow.console) { inspectedWindow.console._commandLineAPI = new CommandLineAPI(this._commandLineAPIImpl, isEvalOnCallFrame ? object : null); expression = "with ((window && window.console && window.console._commandLineAPI) || {}) {\n" + expression + "\n}"; } var result = evalFunction.call(object, expression); if (objectGroup === "console") this._lastResult = result; return result; } finally { if (injectCommandLineAPI && inspectedWindow.console) delete inspectedWindow.console._commandLineAPI; } </code></pre> <p>So basically, it performs a <code>call</code> on the object with the expression. The expression being:</p> <pre><code>with ((window && window.console && window.console._commandLineAPI) || {}) { {}+{};// <-- This is your code } </code></pre> <p>So, as you can see, the expression is being evaluted directly, without the wrapping parenthesis.</p> <h2>Why Node.js acts differently</h2> <p>Node.js's source justifies this:</p> <pre><code>// This catches '{a : 1}' properly. </code></pre> <p>Node did not always act like this. Here is <a href="https://github.com/joyent/node/commit/b45698e67629a894cc005efba5c39d0e8adae1b0#lib/repl.js" rel="nofollow noreferrer">the actual commit that changed it</a>. Ryan left the following comment on the change: "Improve how REPL commands are evaled" with an example of the difference.</p> <hr> <h3>Rhino</h3> <p>Update - OP was interested in how <strong>Rhino</strong> behaves (and why it behaves like the Chrome devtools and unlike nodejs).</p> <p>Rhino uses a completely different JS engine unlike the Chrome developer tools and Node.js's REPL which both use V8.</p> <p>Here is the basic pipe line of what happens when you eval a JavaScript command with Rhino in the Rhino shell.</p> <ul> <li><p>The shell runs <a href="https://github.com/mozilla/rhino/blob/master/toolsrc/org/mozilla/javascript/tools/shell/Main.java" rel="nofollow noreferrer"><code>org.mozilla.javascript.tools.shell.main</code></a>.</p></li> <li><p>In turn, it calls <a href="https://github.com/mozilla/rhino/blob/master/toolsrc/org/mozilla/javascript/tools/shell/Main.java#L340" rel="nofollow noreferrer">this</a> <code>new IProxy(IProxy.EVAL_INLINE_SCRIPT);</code> for example, if the code was passed directly with the inline switch -e.</p></li> <li><p>This hits IProxy's <a href="https://github.com/mozilla/rhino/blob/master/toolsrc/org/mozilla/javascript/tools/shell/Main.java#L102" rel="nofollow noreferrer"><code>run</code></a> method.</p></li> <li><p>It invokes <a href="https://github.com/mozilla/rhino/blob/master/toolsrc/org/mozilla/javascript/tools/shell/Main.java#L196-L202" rel="nofollow noreferrer"><code>evalInlineScript</code></a> (<a href="http://paste.ubuntu.com/5801684/" rel="nofollow noreferrer">src</a>). This simply compiles the string and evals it.</p></li> </ul> <p>Basically:</p> <pre><code>Script script = cx.compileString(scriptText, "<command>", 1, null); if (script != null) { script.exec(cx, getShellScope()); // <- just an eval } </code></pre> <p>Out of the three, Rhino's shell is the one that does the closest thing to an actual <code>eval</code> without any wrapping. Rhino's is the closest to an actual <code>eval()</code> statement and you can expect it to behave exactly like <code>eval</code> would.</p>

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