What is the purpose of adding ( and ) around the code when using eval?
var strJson = eval("(" + $("#status").val().replace(";","") + ")");
Note: The result of $("#status").val() is similar to
{"10000048":"1","25000175":"2","25000268":"3"};The content within that element determines the outcome, and it is best to enclose it in parentheses as a precaution for situations where there may be no input present.
Update: Upon clarification that the content is JSON, the following code is invalid:
eval('{"10000048":"1","25000175":"2","25000268":"3"}');
However, this modification will return a valid object:
eval('({"10000048":"1","25000175":"2","25000268":"3"})');
//effectively:
eval('return {"10000048":"1","25000175":"2","25000268":"3"};');
If included in a JavaScript file:
<script type="text/javascript">
{"10000048":"1","25000175":"2","25000268":"3"}
</script>
Inline object declaration like this will cause failure due to improper syntax, similar to why JSONP requires server support to function correctly.
This deviates slightly from the original question, but since jQuery is being utilized, it would be more efficient to use $.parseJSON() (which utilizes native JSON.parse() if available) like so:
var strJson = $.parseJSON($("#status").val().replace(";",""));
eval is used to evaluate a JavaScript statement or expression. However, {...} can be valid as both a statement and an expression, but the JavaScript grammar prefers it as a statement.
When used as an expression:
{"10000048":"1","25000175":"2","25000268":"3"}
it represents an Object with properties (which is the desired outcome).
On the other hand, when used as a statement, it becomes a block:
{ // begin Block
"10000048": // LabelledStatement (quotes are invalid)
"1", // Expression that calculates string "1" then discards it, followed by
"25000175": // an attempt to put a label inside an expression
This results in an error.
(JavaScript labels are used to label specific statements for use with break/continue, but they are rarely utilized.)
To resolve this ambiguity, adding parentheses clarifies the context. Since only an expression can start with (, the content inside is parsed as an expression, resulting in an object literal instead of a statement context.
It's worth noting that simply adding parentheses may not correctly interpret all JSON values. Due to a design oversight in JSON, the characters U+2028 and U+2029 (obscure Unicode line-ending characters) can be unescaped in a JSON string literal but not in a JavaScript string literal. To ensure safety, you can escape them like so:
function parseJSON(s) {
if ('JSON' in window) return JSON.parse(s);
return eval('('+s.replace(/\u2028/g, '\\u2028').replace(/\u2029/g, '\\u2029')+')');
}
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