Require the JSON library, and use to_json.
require 'json'
tempHash = {
"key_a" => "val_a",
"key_b" => "val_b"
}
File.open("public/temp.json","w") do |f|
f.write(tempHash.to_json)
end
Your temp.json file now looks like:
{"key_a":"val_a","key_b":"val_b"}