Google Apps Script and JavaScript Arrays. Method .filter()

Short description of the method

The .filter() method sequentially compares all elements of the array with the filter condition specified in the callback function. That is, the method, in fact, is a for cycle, inside which the callback function sequentially processes all elements of the array.

The .filter() method can pass three parameters to a callback function:

1. the current value of the array element item
2. optional parameter index - index of the current element
3. optional and rarely used parameter array

In order for an element satisfying the filter condition to be selected, the callback function must return the value true .

All elements for which the callback function returned the value true are collected in a separate array, which can be stored in a new array (in the examples below it is an array result2 ).

In the three variants of the example presented below (examples #1, #2, #3), the same filter condition selects all array elements with a value > 3.

In the last example (the most optimal in terms of code brevity), the logical expression instead of the if operator is set directly to return . Since, as you know, a logical expression can take only 2 values: true или false.

const arr = [1, 2, 9, 4, 1, 6, 5];
    
    let result = arr.filter((item, idx, arr) => console.log(item, idx, arr));
//  [20-06-15 14:53:21:082 EEST] 1 0 [ 1, 2, 9, 4, 1, 6, 5 ]
//  [20-06-15 14:53:21:084 EEST] 2 1 [ 1, 2, 9, 4, 1, 6, 5 ]
//  [20-06-15 14:53:21:085 EEST] 9 2 [ 1, 2, 9, 4, 1, 6, 5 ]
//  [20-06-15 14:53:21:086 EEST] 4 3 [ 1, 2, 9, 4, 1, 6, 5 ]
//  [20-06-15 14:53:21:087 EEST] 1 4 [ 1, 2, 9, 4, 1, 6, 5 ]
//  [20-06-15 14:53:21:089 EEST] 6 5 [ 1, 2, 9, 4, 1, 6, 5 ]
//  [20-06-15 14:53:21:090 EEST] 5 6 [ 1, 2, 9, 4, 1, 6, 5 ]
    
    console.log(result);
//  [20-06-15 14:53:21:091 EEST] []  

// variant #1
  let result2 = arr.filter(item => {
      if (item > 3) {
        return true;
      } else {
        return false;
      };
    });
    
// variant #2
    result2 = arr.filter(item => {
      return item > 3;
    });


// variant #3
    result2 = arr.filter(item => item > 3);
    
    console.log(result2); // [ 9, 4, 6, 5 ]
    
    var sf = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
    let data = sf.getDataRange().getValues().slice(1);
    
    console.log(data);
//  [ [ '', false, 'apples', 5 ],
//    [ '', true, 'carrots', 12 ],
//    [ '', false, 'grapes', 4 ],
//    [ '', false, 'plums', 3 ],
//    [ '', true, 'strawberry', 9 ],
//    [ '', false, 'perches', 4 ],
//    [ '', false, 'bananas', 1 ] ]   

    let newData = data.filter(item => item[1]);

    console.log(newData); 
// [ [ '', true, 'carrots', 12 ], 
//   [ '', true, 'strawberry', 9 ] ]

And another useful script that was not included in this video is a script that filters ONLY unique values.

The idea is very simple: for each element of the array, its index is found and compared with the current index of the value of this element in the array. If the element has not been encountered earlier, then the values of the indices will be equal, and the function uniqValue will return true .

In the first example, for a better understanding of the algorithm, the uniqValue function is written separately. In the second, it is directly "embedded" into the .filter method

let arrayNotUniq = [1, 5, 9, 5];

//variant #1

function uniqValues(item, index, arr) {
  return arr.indexOf(item) === index;
}

let arrayUniq = arrayNotUniq.filter(uniqValues)                        // [1, 5, 9]



//variant #2

  let arrayUniq = arrayNotUniq.filter((item, index, arr) => {
    return arr.indexOf(item) === index;
  });                                                                                                     // [1, 5, 9]

You can find more information in this video (RU voice):