In one my script tag, I am having
pt=[]
pt[1]= JSON.parse('{"AB":"1","BC":"1.309","XY":"0.40"}');

But, when I am previewing question, I am getting Uncaught SyntaxError: missing ) after argument list. When I click error on console, it shows pt[1]= JSON.parse('<span class='em-expression em-haveerror'><span title='' class='em-var-string'>"AB"</span><span title='Unsupported syntax' class='em-error'> : </span><span title='' class='em-var-string'>"1"</span>, <span title='' class='em-var-string'>"BC"</span><span title='Unsupported syntax' class='em-error'> : </span><span title='' class='em-var-string'>"1.309"</span>, <span title='' class='em-var-string'>"XY"</span><span title='Unsupported syntax' class='em-error'> : </span><span title='' class='em-var-string'>"0.40"</span></span>');

I checked the same code works in javascript console. What could be the problem. How I can use JSON objects in limesurvey script tag

I have to escape curly braces in order to use JSON.parse.

pt[1]= JSON.parse('\{"AB":"1","BC":"1.309","XY":"0.40"\}');

If you use {something} inside a question : Expression Manager treat it.

To disable : just add a space : { something }

No need to escape, but escaping disable too Expression Manager

That is correct, you have to escape the curly braces.
Alternatively, what shnoulle wrote is also correct, a simple whitespace will also prevent the EM from parsing it.
But some functionality isn't well working with whitespaces, so sometimes, you will need to escape.

markusfluer wrote: But some functionality isn't well working with whitespaces, so sometimes, you will need to escape.

? Do you have an example ? Json with or without space are same , javascript with or without space are same too, no ?

If you just need to show curly brace to user (for example : make a question about Expression manager) : use html entities

Example

Did this code is valid in all surveys : &amp;#123;TOKEN:TOKEN&amp;#125;