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;

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

    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));

<!-- end snippet -->