the latter element subtract the previous element in javascript

December 12, 2021

There is an array:

let arr=[
    [1000,800,1,"true"],
    [1500,0,2,"false"],
    [1600,0,3,"true"],
    [2500,300,4,"false"]
]

I want the result:

let arr_result=[
    [1000,800,1,"true"],
    [500,0,2,"false"],
    [100,0,3,"true"],
    [900,300,4,"false"]
]

That is, let the latter sub-array element[0] subtract the previous sub-array element[0].
I need to do it in javascript.

How to do it?

>Solution :

You can use map method and then simply use index param to get previous element by using array[index - 1] and then first element of that sub array.

let arr = [
  [1000, 800, 1, "true"],
  [1500, 0, 2, "false"],
  [1600, 0, 3, "true"],
  [2500, 300, 4, "false"]
]

const result = arr.map(([e, ...rest], i) => (
  [i ? e - arr[i - 1][0] : e, ...rest]
))

console.log(result)