how to type various arguments?

For creating a tuple, you just put the types inside of the square brackets in the order that they appear. [string, string[]] is the type for a 2-tuple of a string and an array of strings (string[]).

Since we are spreading the arguments of runProcess and treating it like an array, we need to treat the other argument type – a single string – as a 1-tuple [string].

We say that our args proc is either one of these two tuples: [string] | [string, string[]]. Therefore we know that the first element in the args array is always a string and the second element is either string[] or undefined. In order to call child_process.spawn, we want to default to an empty array for the second argument if none is given.

That signature looks like:

const runProcess = (...proc: [string] | [string, string[]]) => {
    const [str, arr = []] = proc;
    const child = child_process.spawn(str, arr, {
        stdio: ["ignore", "pipe", "pipe"],
    });
}

But given that we are always dealing with exactly one or two arguments, I’m not sure that it really makes sense to spread the arguments as ...proc. Why not just have a required first argument and an optional second argument that defaults to an empty array?

const runProcess = (command: string, args: string[] = []) => {
    const proc = child_process.spawn(command, args, {
        stdio: ["ignore", "pipe", "pipe"],
    });
}

Playground Link

Edit: as suggested by @Aleksey L., you can pass the spawn args as individual string arguments rather than as an array.

const runProcess = (command: string, ...args: string[]) => {
    const proc = child_process.spawn(command, args, {
        stdio: ["ignore", "pipe", "pipe"],
    });
}

Playground Link