为什么要用eval("({a:'b'})")来将字符串转换成json对象

For the same reson as:

{a: 'abc', b: 'def'};

causes an error but:

( {a: 'abc', b: 'def'} );

does not.

When encountered on the left hand side, the punctuator '{' defines the
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 the
right hand side of an expression, in which case {} is treated as an
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.