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Webmaster Forum / HTML, CSS, Scripts / JavaScript / July 2006Help me understand this JSON eval problem please
Why is this: eval('[{\"active":"true"}]') is OK, but eval('{"active":"true"}') tells me I'm missing ";" ? Then why is it that if I put parentheses around the second statement: eval('({"active":"true"})') Then it's OK again? Thanks in advance! Ray > Why is this: > [quoted text clipped - 12 lines] > Thanks in advance! > Ray Because: [{"active":"true"}] is a valid expression, but {"active":"true"} is a syntax error at the colon. The first one defines an array containing an object, in the second one the '{' is starting a block, not defining an object. See the Ecmascript language specification, section 12.4 Expression statement: > Note that 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. > Why is this: > [quoted text clipped - 9 lines] > > Then it's OK again? A javascript Expression Statement may not commence with an opening brace, so that it cannot be ambiguous along side Block Statements. The - eval - function is supposed to execute a string as if it satisfied the production rules for a Program, and Programs are made up of statements, so - eval - must see the first opening brace and starting a Block statement, and then the absence of a semicolon before the colon is a syntax error. With the parentheses the opening brace is becomes part of a parenthesised Expression and so must be interpreted as the start of an object literal, and the parenthesised Expression becomes an Expression Statement (as no other production can put an opening parenthesis at the start of a Statement). Richard. Thanks Duncan, Richard, for the explanations! Cheers, Ray > Why is this: > [quoted text clipped - 12 lines] > Thanks in advance! > Ray |
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