The code is find the max value

1. How this code work

// this is original code
const items = [
    { id: 0 },
    { id: 1 },
    { id: 2 }
]

function id(items) {
    const maxId = items.reduce((maxId, items) => 
        Math.max(items.id, maxId), -1
    )
    return maxId + 1
}
console.log(id(items))  // I get 3

The reduce methods callback (maxId, items) => Math.max(items.id, maxId), -1

I can not figure out what is this, it just return something, -1

I never seem this syntax ever.

A comma between two value, what I expect is return [ , ] or ( , )

2. Why this code get different result

I also tried this below, I think its same code above. But result is not same.

// I changed something, I add { return  something in reduce callback }
const items = [
    { id: 0 },
    { id: 1 },
    { id: 2 }
]

function id(items) {
    const maxId = items.reduce((maxId, items) => {   // Here
            return Math.max(items.id, maxId), -1     // add
        }                                            // { return }
    )
    return maxId + 1
}
console.log(id(items))  // I get 0