Looping through an array is a very important trick to learn in Apps Script. In this example we will select a range, where each cell contains a number, then we will loop trough all the values and sum them up.

Let’s select the range first

We are selecting the range A1 to A10 in Sheet1 where we manually input some random numbers.

function sum() {
  var ss = SpreadsheetApp.getActive();
  var s = ss.getSheetByName("Sheet1");
  var rg = s.getRange("A1:A10");
}

Looping through all the values within the range

The getValues function automatically extract all the values within the selected range and save the result as an array. Using the forEach function we are looping through the array and extracting the content of each cell one by one. We will also have to convert each value from string to integer with Number so we can sum them up. Finally we will paste the result in cell B1:

function sum() {
  var ss = SpreadsheetApp.getActive();
  var s = ss.getSheetByName("Sheet1");
  var vals = s.getRange("A1:A10").getValues();

  var result = 0;

  vals.forEach(function(v) {
    result = result + Number(v);
  });
  
  s.getRange("B1").setValue(result);
}

Et voila!