Why does the Google Chrome console throw a "SyntaxError: Unexpected token" when entering (

Question

In the Google Chrome console as you type

( 

and Enter, Chrome says "SyntaxError: Unexpected token" "Why? Input is just" (", including no"} ".

We get the same error when we enter

 console.log( 

Not "}"!!!

The next token should be a list of arguments or ")", so the error message should be "List of expected arguments" or "Unblocked (" or something ".

And I want to know if console input is being entered as StatementList(opt) (defined in ECMA-262)?

Answer 1

Edit: I found the exact code that is being evaluated. The code is in "src / third_party / WebKit / Source / WebCore / inspector / InjectedScriptSource.js".

Before the Chrome console evaluates your code, it completes it in a with block to include command-line features in scope. So, what you type is actually evaluated inside braces. An unexpected "}" token is one that is automatically placed in Chrome.

The code that Chrome passes to eval is

 with ((window && window.console && window.console._commandLineAPI) || {}) { <your code here> }; 

Since this is a simple text substitution, the following example works, and the result is an object that you can expand to see the answer property:

 } 0, { answer: 42 

Which (reformatted) is equivalent:

 with ((window && window.console && window.console._commandLineAPI) || {}) { } 0, { answer: 42 }; 

} at the beginning closes the with block. Part 0, needed to force the object literal to be parsed as an expression instead of another block. Then { answer: 42 is the beginning of the object literal, which is closed by the inserted marker } .

For more fun, here are some other inputs that work (and their results):

 > }{ // an empty block, so no value undefined > }!{ // !{} === false false > }!!{ // !!{} === true true > } +{ valueOf: function() { return 123; } 123