Don't understand why you should not use an object literal at the beginning of a statement

Question

Just reading some of the JS tuts on Mozilla and came across the statement "You should not use an object literal at the beginning of a statement. This will lead to an error or not behave as you expect, because the { will be interpreted as the beginning of a block."

I don't understand what they mean. Could someone shed some light on this please

Assume this is the link in question: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Values,_variables,_and_literals#Object_literals — Matt Burland, Nov 18 '14 at 15:23
Example of error: `{ test: 'test' } === 'test';` Then no error: `'test' === { test: 'test' };` — David Sherret, Nov 18 '14 at 15:28
@DavidSherret: You should consider making that into an answer. Paul's answer is ok, but I think this is a better example of an actual error. — Matt Burland, Nov 18 '14 at 16:06
@MattBurland I thought about it, but how many people actually write conditions without assigning them to a variable or putting them in an if statement? At the moment, I can't think of a better example though. Feel free to post an answer using my example if you'd like. — David Sherret, Nov 18 '14 at 16:17
@DavidSherret: Agreed. It seems you need something really contrived before this even really becomes an issue. So why MDN is correct that it *could* be an issue, it seems *really* unlikely to be something that would trip you up in normal coding. — Matt Burland, Nov 18 '14 at 16:21
@MattBurland Exactly. I've never seen anyone ask the question "why isn't this code working" with this being the issue. Maybe someone has though... — David Sherret, Nov 18 '14 at 17:10
@MattBurland I came across a decent example today of when this might trip someone up (ES6): `var a, b, c; {a, b, c } = { a:1, b:2, c:3 };` http://stackoverflow.com/q/27864423/188246 — David Sherret, Jan 09 '15 at 17:57

score 3 · Answer 1 · answered Nov 18 '14 at 15:25

3

An object literal starts with {

{ name: "Paul" age: 30 } // I wish

But so does a block.

{ if (age < 30) console.log ("He's lying again"); }

When the interpreter sees "{" it has to pick one interpretation (*). It picks "block" and tries to parse your object literal as if it was code. Which it isn't and so things quickly go wrong.

(*) Well, it doesn't have to, it could look ahead to see if the rest of the block looks like an object literal or code, but disambiguating the two would make the parser a lot more complicated, so the language is defined so it doesn't have to do this.

answered Nov 18 '14 at 15:25

The Archetypal Paul

41,321
20
104
134

Good answer with a bit of humour :) – dayuloli Nov 18 '14 at 15:34
Thanks everyone for their input, much appreciated – user2366825 Nov 18 '14 at 16:25

Don't understand why you should not use an object literal at the beginning of a statement

1 Answers1

Linked

Related