What is the rationale behind this particular syntax structure?
eval('(' + jsonString+ ')')
When it comes to parsing JSON text, Crockford explains that "The text must be wrapped in parentheses to prevent any confusion with JavaScript's syntax." Learn more about this explanation here. But how can we circumvent this requirement?
Is there a way to bypass it?
In discussing syntax ambiguity, Crockford points out that in certain contexts, an open curly brace may be mistakenly recognized as the beginning of a block instead of an object literal.
For instance:
{"foo": "bar"} // SyntaxError
This would result in a syntax error because it is interpreted as a block with a string literal "foo" and an unexpected use of the token :.
On the contrary, using parentheses, known as the grouping operator, eliminates this ambiguity since blocks are only expected in a statement context.
({"foo": "bar"})
Edit: @el.pescado raises an interesting question:
Can you explain why eval('{}') is undefined?
ECMAScript defines a Completion Specification Type to describe statement behaviors, consisting of triples like (type, value, target) where type can be normal, break, continue, return, or throw.
For an empty block (Block : {}), the explicit completion is:
Return (normal, empty, empty).
After executing code, the eval function checks the result completion. If the type is normal and value is empty, it returns undefined according to Step 7.
...
7- If result.type is normal and its completion value is empty, then return the value undefined.
...
To properly evaluate an object literal in the eval context and other contexts, it must be wrapped in parentheses:
For example, eval('{}') evaluates to undefined, but eval('(' + '{}' + ')' ) evaluates to Object.
If you try entering {"foo":"bar"} directly into the console, it will throw an invalid label error. However, by wrapping it in parentheses, it becomes a valid expression.
@fisherman, the code that is evaluated by eval() must be written in a way that JavaScript can understand. For example, if you assign the following string to a variable:
eval(' var foo1 = {"foo" : "bar"}'); foo1.foo will return bar
Therefore, my understanding is that since there is no valid statement starting with "{" in JavaScript, it is causing an error.
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