Why the open quote and bracket for eval('(' + jsonString+ ')') when parsing json string
eval, javascript, json
Solution
The syntax ambiguity to which Crockford refers is that if an open curly brace is not found on expression context, it will be recognized like a block, and not like the start of an object literal.
For example:
{"foo": "bar"} // SyntaxError
Will give you a syntax error, because it will be interpreted as a block, with a string literal "foo", and a unexpected usage of the token `:`.
On the other hand, the parentheses, formally called the grouping operator, can only evaluate expressions, therefore we will not have any syntax ambiguity because a block can only be expected on a statement context.
({"foo": "bar"})
Edit: @el.pescado makes an interesting question:
Can you explain why eval('{}') is undefined?
ECMAScript describes an internal type to explain the behavior of statements, it's called The Completion Specification Type.
Values of the Completion type are triples of the form of `(type, value, target)`, where `type` can be `normal`, `break`, `continue`, `return`, or `throw`.
`value` can be any language value or `empty`, and `target` any Identifier or `empty`.
An empty block (the production `Block : {}`) explicitly returns the following completion:
Return (normal, empty, empty).
The `eval` function, after executing the code, and exiting the newly created execution context, checks the result completion of the evaluated code, and in the Step 7 we can see that `undefined` is explicitly returned if the completion type is `normal` and the completion value is `empty`:
...
7- If result.type is normal and its completion value is empty, then return the value undefined.
...
Problem
Can you please tell me the reason for this specific syntax structure ``` eval('(' + jsonString+ ')') ``` When parsing json text. Crockford says "The text must be wrapped in parens to avoid tripping on an ambiguity in JavaScript's syntax." here. What does that mean? Can we avoid it?