If the arrow function does not use curly braces{}, it is equivalent to returning directly

const arr = [1, 2, 3, 4] arr.map(i => i) // 1 ,2 ,3, 4

But if I want to return an object, I cannot use the simplest form?

arr.map(i => { a: i}) // [undefined, undefined, undefined, undefined]

Must be enclosed in curly braces

arr.map(i => return {{ a: i}}) // [Object, Object, Object, Object]