Add all elements from a table to another table in lua

Question

In javascript you can do the following

const obj1 = {a:1, b:'blue', c:true};
const obj2 = {...obj1, d:3, e:'something else'};

and you object 2 will have all of obj 1 in it.

Is there a way to do this in lua?

Answer 1

You'd have to write your own function for this purpose:

local function add_missing(dst, src)
    for k, v in pairs(src) do
        if dst[k] == nil then
           dst[k] = v
        end
    end
    return dst -- for convenience (chaining)
end

which you may then use as follows:

local obj1 <const> = {a = 1, b = 'blue', c = true}
local obj2 <const> = add_missing({d = 3, e = 'something else'}, {a = 1, b = 'blue', c = true})

---

Note: JavaScript's spread operator also guarantees a certain order of execution; that is, in your example fields from obj2 would override those from obj1, which is why I've included an explicit nil check to only include missing fields:

> {...{a: 2}, a: 1}
{ a: 1 }
> {a: 1, ...{a: 2}}
{ a: 2 }