Wolfram Function Repository
Instantuse addon functions for the Wolfram Language
Function Repository Resource:
An operator that joins in front of its arguments the sequence of lists it is provided
ResourceFunction["JoinMost"][list] an operator that will join the sequence of lists it encounters with list. 
JoinMost uses Join to join its argument {3,4} with whatever else it encounters, which one may think of as Most of the Join:
In[1]:= 

Out[1]= 

JoinMost when used with an association as its argument will override keyvalue pairs from the sequence of lists it encounters if any of them have the same key:
In[2]:= 

Out[2]= 

The operator created by JoinRest using an association can accept a sequence of associations and will override keyvalue pairs if and only if it encounters any association in the sequence with the same key:
In[3]:= 

Out[3]= 

If JoinMost has no argument, it operates as an identity on the data it is given:
In[4]:= 

Out[4]= 

If JoinMost has a sequence of associations as its argument, it will use the last value for a key it confronts in that sequence to perform the override:
In[5]:= 

Out[5]= 

JoinRest can be right composed with Counts to create an Association that coerces an answer for certain anticipated values:
In[6]:= 

Out[6]= 

JoinMost works similarly to Merge with a Last combiner function:
In[7]:= 

Out[7]= 

JoinMost is very similar to the resource function JoinRest. In JoinMost, its argument is placed last in the Join; in JoinRest, its argument is placed first in the Join.
JoinMost, like Join, will fail to evaluate if the heads of the expressions to be joined differ from one another:
In[8]:= 

Out[8]= 

Why you should read code carefully before trusting the results :
In[9]:= 

Out[9]= 

This work is licensed under a Creative Commons Attribution 4.0 International License