One possible solution is slice the array (to remove the two first elements) and then map, transform the numbers from string to number:

let array = ['1','2','3','4','5','6']
let result = array.slice(2, array.length)
let numbers = result.map(el=>Number(el))
console.log(numbers)

You can choose to use splice, but you don't want the result. You just want to run that to remove the first 2 values. Then map the remaining values to an array of numbers and you're good.

let array = ['1','2','3','4','5','6'];
array.splice(0, 2); // splice will remove indexes 0 and 1
let result = array.map(n => +(n)); // map the remaining values to numbers
console.log(result);

The Number constructor converts one value to a number - Number(value)

You're putting in as value a spreaded array from which the first value is taken and converted

Number(...array.splice(2))  // Number('3','4','5','6') => 3

I don't see any good example using the spread operator, since you don't want to get rid of the array structure, but have to iterate the array for converting each element from string to number

1. For getting rid of the first two entries without altering the original array use .slice() The second argument can be omitted if you want to slice from the index to the end of the array
2. To keep it short you can directly map on the spliced array using either the Number(v) constructor or by simply adding the + at the front

let array = ['1','2','3','4','5','6']
let numbers1 = array.slice(2).map(str => +str) // [3, 4, 5, 6]
let numbers2 = array.slice(2).map(str => Number(str)) // [3, 4, 5, 6]