JSErrorReport
jsreport_strict_mode_error this condition is an error in strict mode code, a warning if js_has_strict_option(cx), and otherwise should not be reported at all.
... jsreport_is_strict_mode_error(flags) returns true if flags has jsreport_strict_mode_error.

JSAPI reference
reporterrornumberuc js_reporterrorflagsandnumber js_reporterrorflagsandnumberuc js_reporterrornumberucarray added in spidermonkey 24 js_reportoutofmemory js_reportallocationoverflow added in spidermonkey 1.8 js_geterrorreporter js_seterrorreporterobsolete since jsapi 52 js_errorfromexception js_geterrorprototype jsreport_is_exception jsreport_is_strict jsreport_is_warning jsreport_is_strict_mode_error the following functions allow c/c++ functions to throw and catch javascript exceptions: js::createerror added in spidermonkey 38 js_isexceptionpending js_getpendingexception js_setpendingexception js_clearpendingexception js_throwstopiteration added in spidermonkey 1.8 js_isstopiteration added in spidermonkey 31 typedef jsexceptionstate js_saveexceptionstate js_restoreexcep...

Components.utils
strict_mode boolean read only.

TypeError: can't assign to property "x" on "y": not an object - JavaScript
in strict_mode, a typeerror is raised when attempting to create a property on primitive value such as a symbol, a string, a number or a boolean.