A short review;

• You use var for the function, I would use const, actually I would just use function ;)
• I would use a n inline function that can process a window, otherwise it's hard to tell that you implemented a sliding window
• I would avoid the word array in variable names, unless they are an array

Counter-proposal;

function sumSubArrays(list, k){

  const sumWindow = (list) => list.reduce((a, b) => a + b, 0);

  let sum = 0;
  for(let i = 0; i <= list.length-k; i++){
    sum += sumWindow( list.slice(i, i + k) );  
  } 
  return sum;
}



let arr = [1,2,3,4,5]
let k = 3;

console.log(27, 27 === sumSubArrays(arr, k));

A short review;

• You use var for the function, I would use const, actually I would just use function ;)
• I would use a n inline function that can process a window, otherwise it's hard to tell that you implemented a sliding window
• I would avoid the word array in variable names, unless they are an array
• You never declare current, making it a global

Counter-proposal;

function sumSubArrays(list, k){

  const sumWindow = (list) => list.reduce((a, b) => a + b, 0);

  let sum = 0;
  for(let i = 0; i <= list.length-k; i++){
    sum += sumWindow( list.slice(i, i + k) );  
  } 
  return sum;
}



let arr = [1,2,3,4,5]
let k = 3;

console.log(27, 27 === sumSubArrays(arr, k));

A short review;

• You use var for the function, I would use const, actually I would just use function ;)
• I would use a n inline function that can process a window, otherwise it's hard to tell that you implemented a sliding window
• I would avoid the word array in variable names, unless they are an array

Counter-proposal;

function sumSubArrays(list, k){

  const sumWindow = (list) => list.reduce((a, b) => a + b, 0);

  let sum = 0;
  for(let i = 0; i <= list.length-k; i++){
    sum += sumWindow( list.slice(i, i + k) );  
  } 
  return sum;
}



let arr = [1,2,3,4,5]
let k = 3;

console.log(27, sumSubArrays(arr, k));

A short review;

• You use var for the function, I would use const, actually I would just use function ;)
• I would use a n inline function that can process a window, otherwise it's hard to tell that you implemented a sliding window
• I would avoid the word array in variable names, unless they are an array

Counter-proposal;

function sumSubArrays(list, k){

  const sumWindow = (list) => list.reduce((a, b) => a + b, 0);

  let sum = 0;
  for(let i = 0; i <= list.length-k; i++){
    sum += sumWindow( list.slice(i, i + k) );  
  } 
  return sum;
}



let arr = [1,2,3,4,5]
let k = 3;

console.log(27, 27 === sumSubArrays(arr, k));