How do you write 'plus equals' in Lua for a vim setting?

Question

How does set mouse+=a translate to the equivalent Lua Neovim setting?

Answer 1

vim.opt.mouse returns a mouse option object.

You can use Option:append(value) to append a string value to an option like this.

-- These are equivalent
vim.opt.mouse:append('a')
vim.opt.mouse = vim.opt.mouse + 'a'

Learn more in :h vim.opt:append()

There is also a vim.o.mouse, which returns the option as a string. You can also append a string value to it using ...

vim.o.mouse = vim.o.mouse .. 'a'