CODEWITHSUNDEEP
javascriptnode.js
user8273038
user8273038

Reputation: 43

evaluating "()" in node.js causes wierd behaviour, but browser says syntax error (bug in node.js?)

When I evaluate the expression "()" in node.js (without the quotes), it corrupts the environment. The global object is trashed and nothing works. When i evaluate the same express in the browser console it reports a syntax error. From my research it appears the browser is correct. Is this a bug in the node.js expression parser? Can anyone explain what is happening under the hood? Here is a screenshot

Upvotes: 4

Views: 66

Answers (3)

randomir
randomir

Reputation: 18697

Imagine you're writing your own JavaScript REPL. You know JavaScript's grammar in and out, and you want to abort processing of an invalid input as soon as possible (but NOT before that). So, you decide to read character by character, build token by token and abort as soon as you encounter the impossible token (for a current state, state defined by input processed thus far).

So, you know if users enters (}, it's pretty safe to abort - there's no way something meaningful is going to come out of that!

Also, if user enters (2, you don't want to abort yet - there are a plenty of valid characters to follow, like ), *, ,, etc.

You also know what to expect after (). If it isn't =>... (lambda expression), than it's invalid token.

Now back to reality. REPL implementations in Node.js, Chrome, Firefox, etc. differ because they can. Some are "smarter" than other, and will abort sooner than others, but that's really up to them.

Node.js chose to have "less smart" REPL, as it won't abort reading your input, as soon as it can. In fact, if you entered something that can never form a valid expression, it will leave you hanging, reading junk until you explicitly abort with Ctrl+C.

Chrome's console is a little bit smarter. When you type (), you'll get:

> ()
Uncaught SyntaxError: Unexpected token )

but, when you start typing a lambda ()=>, it will allow you to finish it in the next line:

> ()=>
2

As a comfort note, you'll get the same error in Node.js REPL if you indicate you're done with entering of your expression, like this:

> eval("()")
SyntaxError: Unexpected token )

Upvotes: 1

joews
joews

Reputation: 30330

() by itself isn't valid JavaScript syntax.

When Node encounters a Syntax Error in the REPL it waits for more input. It tries to indicate this by changing your prompt to three dots.

This is useful because sometimes you want to enter some code that contains a new line:

> () => { // this isn't valid syntax by itself
... }     // but you can complete a valid expression like this

In this case there is no input you could add to complete a valid expression, but you can exit the "waiting for input" mode with ctrl-c.

Upvotes: 3

Nat Mote
Nat Mote

Reputation: 4086

This is just what happens when Node can't parse the expression you have typed, and it's waiting for more input to see if it can complete the parse. Try any other nonsense like ((( ), etc. and you will get the same result. You can use Ctrl-C to abort the parse and get back to a working prompt.

Upvotes: 0

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