Regular anonymous function:

let numbers = [10, 100, 1000]

let doubled = numbers.map(function(x){
  return x * 2
})

document.write(doubled)

Alternative arrow function:

let doubled = numbers.map((x) => { return x * 2 })

If it's only a single line we don't need curly brackets and the keyword

return

let doubled = numbers.map((x) => x * 2);

If the arrow function has only one parameter, it does not need parentheses:

let doubled = numbers.map(x => x * 2);

Note: We only need

parentheses

if we had zero or more than one parameters