Why use eval ("({A: 'B'})") to convert a string to a JSON object?

For the same reson:

{A: 'abc', B: 'def '};

Causes an error:

({A: 'abc', B: 'def '});

Does not.

When encountered on the left hand side, the punctuator '{'defines
Start of a block statement, like if {... or while {... etc. The stuff
Inside the block is evaluated as if it were a series of statements, so
The script engine attempts to evaluate:

A: 'abc', B: 'def'

And barfs (understandably ).

By enclosing the expression in () It is evaluated as if it were
Right hand side of an expression, in which case {} is treated as
Object initialiser. So what you must pass to Eval is:

({/* Property names & values */})

As a literal string to force it to treat the {} as an object
Initialiser.