CODEWITHSUNDEEP
javascriptgoogle-chrome
bcoughlan
bcoughlan

Reputation: 26627

Chrome - JSON object statements unusual behaviour in console

I'm playing with JSON objects in the Chrome console and I came across this unusual behaviour:

> {a:1}
1
> {"a":1}
SyntaxError: Unexpected token :
> b={a:1}
Object
> b={"a":1}
Object

Why does the first statement return 1 instead of an object, and why does the second statement work? I expected the first two statements to return the same output as the last two statements.

Upvotes: 3

Views: 147

Answers (3)

I Hate Lazy
I Hate Lazy

Reputation: 48761

A JavaScript expression statement can not start with a { because it would cause an ambiguity for the interpreter, which could also see it as a statement block.

So this is considered a statement block with a statement label and a numeric literal instead of an object literal:

{a:1}

But this is considered a statement block with invalid syntax, since there's no statement that can begin with "a":

{"a":1}

But these don't start with a {. They start with the b = so the { is considered to begin the object literal.

b = {a:1}
b = {"a":1}

12.4 Expression Statement

NOTE An ExpressionStatement cannot start with an opening curly brace because that might make it ambiguous with a Block. Also, an ExpressionStatement cannot start with the function keyword because that might make it ambiguous with a FunctionDeclaration.

This means that all you need to do is start the expression statement with a different character to make it valid.

For example, you could wrap it in parentheses, and it will work:

({"a": 1})

Upvotes: 1

Moritz Roessler
Moritz Roessler

Reputation: 8621

Chrome evaluates console input like this:

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

This leads to the { } brackets seen as extra block scope bracket and therefore getting ignored

Which then leads us to

a:1 // 1

Seen as a label

And

a={a:1}

as correct assignment

Edit:

This is also what JSLint says on this JSBin

Upvotes: 1

jevakallio
jevakallio

Reputation: 35890

My best guess would be that in the first scenario

> {a:1}

The curly braces are being ignored, and the a is interpreted as a label.

You get the same value 1 by simply typing

> a:1

If this is correct, the second example doesn't work because double quotes aren't acceptable characters in a label.

The third and fourth examples work, because they are valid variable assignments, and the console understands the objects as objects.

Upvotes: 1

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