javascript/RuntimeHeap is a SAP Parameter attribute which is used to control CL_JAVA_SCRIPT: memory usage before garbage collection information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
This parameter allows you to enter the lower limit for the memory usage
of JS objects (data, scripts, ...) as of which the JS engine starts
garbage collection. This does not mean that this memory is requested
immediately but that simply a threshold value is agreed for the attempt
to release objects that are no longer used.
CAUTION
A value that is too small can cause the JS Garbage Collector to start
constantly (an intensive procedure) which in turn causes significant
drop in system performance.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.