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Split between two character in javascript

this is my string,
var str=""{\"checkedRows\":\"[{\"SlNo\":\"1\",\"ItemCode\":\"8\",\"ItemDetail\":\"Cassue Nut\",\"Qty\":\"140\",\"Rate\":\"2000\",\"Amount\":280000\"}]\",\"checkedIndex\":\"[0]\"}"";
I'm trying to get substring as below
[{\"SlNo\":\"1\",\"ItemCode\":\"8\",\"ItemDetail\":\"Cassue Nut\",\"Qty\":\"140\",\"Rate\":\"2000\",\"Amount\":280000\"}]
And i'm trying below code to split
var newStr = str.substring(str.lastIndexOf("[") + 1, str.lastIndexOf("]"))
but unable to fetch it.

Firstly note that you have an extra leading and trailing double quote around your JSON string. Secondly the JSON itself is completely malformed. You need to fix the errors it has (extra/mis-matched double quotes mostly) so it looks like this:
'{"checkedRows":[{"SlNo":"1","ItemCode":"8","ItemDetail":"Cassue Nut","Qty":"140","Rate":"2000","Amount":280000}],"checkedIndex":[0]}'
Once you've done that you can parse the string to an object, then retrieve the checkedRows property and stringify that again, like this:
var str = '{"checkedRows":[{"SlNo":"1","ItemCode":"8","ItemDetail":"Cassue Nut","Qty":"140","Rate":"2000","Amount":280000}],"checkedIndex":[0]}';
var obj = JSON.parse(str);
var newStr = JSON.stringify(obj.checkedRows);
console.log(newStr);

How to convert a string value to Json

I receive a string in the format given below (with double quotes around it):
"'{ "param" : "value"}'"
How can I convert this string into json object so that I could use it as
alert(obj.param)
Thank you
Edit
As I mentioned in above the string has double quotes around it. Here is an example of how I get this string
CSHTML
#{
var prop = "{ \"param\" : \"value\"}";
<a data-type="#prop"></a>
}
and in JS I have this
var obj = anchor.data('type');
I want to be able to use obj.param, how do I achieve this?
Thanks

Use JSON.parse it parses a string as a JSON
var obj = JSON.parse(json);

Why is stringified JSON a valid string?

We're already aware that we cannot add double quotes inside the double quotes:
var str = ""hello""; //this will be invalid string
but when I stringify an object like this
var obj = {"name":"abc"}
var str = JSON.stringify(obj).
str // returns "{"name":"abc"}"
which is valid but shouldn't be. I'm confused as in do JavaScript have some special cases when we stringify a JSON object and omit the string validations on it?
Thanks in advance.

You can have as many double quotes inside a string literal as you want. You just need to scape them using a backslash prefix (\" instead of ").
Try this example in your browser console:
var myStr = "\"Hello\"";
myStr
You should see ""Hello"" in your console. That would be how the stringify creates a string with double quotes in it.

JSON.stringify escaping without need

JSON.stringify is converting my json object to the following string
{\"2003\":{\"1\":{\"2\":[\"test\"],\"3\":[\"test2\"]}}}
When it should not be escaped. The result should be as the string quoted below
{"2003":{"1":{"2":["test"],"3":["test2"]}}}
Rather than use a general replace of all the escaped quotes and remove ones that could be in the input. How can I set JSON.stringify to not double escape the variables?

You are stringifying a string, not an object:
var str = '{"2003":{"1":{"2":["test"],"3":["test2"]}}}';
var obj = {"2003":{"1":{"2":["test"],"3":["test2"]}}};
console.log( JSON.stringify(str) ); // {\"2003\":{\"1\":{\"2\":[\"test\"],\"3\":[\"test2\"]}}}
console.log( JSON.stringify(obj) ); // {"2003":{"1":{"2":["test"],"3":["test2"]}}}

Try these two examples in browser`s console:
let obj = {2003:{1:{2:["test"],3:["test2"]}}};
JSON.stringify(obj);
-> "{\"2003\":{\"1\":{\"2\":[\"test\"],\"3\":[\"test2\"]}}}"
and
let obj = {2003:{1:{2:["test"],3:["test2"]}}};
console.log(JSON.stringify(obj));
-> {"2003":{"1":{"2":["test"],"3":["test2"]}}}
In both cases the string returned from JSON.stringify is valid
In the first case you print "raw" string to console which starts and ends with double quote and all nested double quotes need to be escaped (\" instead of "). JSON validators will mark this string as malformed JSON but it is still parsable with JSON.parse
In the second case you print string "interpreted" as JSON by console.log. JSON validators will mark it as valid JSON but it is no parsable with JSON.parse because it is not string (no quotes around)

getting json from string

I have the following json object
"phrase": "{subject: Hello}"
When I access "phrase" it returns "{subject: Hello}" as a string but I want this string to be converted to json object.

There is a function called JSON.parse to convert things from strings to objects but I am not sure it would apply to your case, since you have invalid JSON (the "Hello" not being quoted is a bid deal and the "subject" not being quoted is a bad sign)

If it's a Javascript object literal, just remove the quotation marks when you create it:
var phrase = { subject: "Hello" };
If it's a JSON string that is parsed, change the string to an object:
{ "phrase": { "subject": "Hello" } }
If you have a variable that contains a JSON string, you need to make it valid JSON to parse it:
var phrase = '{ "subject": "Hello" }';
var obj = JSON.parse(phrase);
You can also parse the string as Javascript, which has a more relaxed syntax. The string value needs delimiters though:
var phrase = '{ subject: "Hello" }';
var obj = eval(phrase);
Note that the eval function actually executes the string as javascript, so you need to know where the string value comes from for this to be safe.

Use JSON.parse():
var obj = {myObj:"{\"this\":\"that\"}"};
obj.myObj = JSON.parse(obj.myObj);
alert(obj.myObj["this"]);
Here is the demo.

you could use native JSON parsing with JSON.parse(jsonString);
(Edit: assuming to have a valid JSON object)