Convert json data to comma separated string

I have a json object that has an unknown number of variables, like this:

{ 
  "var1": 123, 
  "var2": 456, 
  "var3": 789 
}

How can I automatically convert it to form the following string?

'var1=123,var2=456,var3=789'

Note, I am looking for a solution that doesn't require installing jq.