Google Sheets macro to center align all dashes in top row - google-apps-script

Question

I'm trying to use a macro to check the top row of my sheet and automatically center align all cells that have only a dash. Right now, it looks like my code is running, but it's not doing anything:
function CenterAlignAllDashes() {
var spreadsheet = SpreadsheetApp.getActive();
var topRow = spreadsheet.getRange('A1:1');
var topRowValues = topRow.getValues();
for (let i=0; i < topRowValues.length; i++) {
if (topRowValues[i] == '-') {
topRow[i].setHorizontalAlignment('center');
}
}
};

Answer 1

Modification points:
In your script, topRowValues is var topRowValues = topRow.getValues();. In this case, it is a 2-dimensional array. By this, topRowValues[i] == '-' is always false.
topRow is var topRow = spreadsheet.getRange('A1:1');. By this, topRow[i].setHorizontalAlignment('center') occurs an error because topRow[i] is undefined. But, by the above issue, the script in the if statement is not run.
About var topRowValues = topRow.getValues() and topRowValues.length, in this case, topRowValues.length is 1. By this, the for loop is looped only one time.
About spreadsheet.getRange('A1:1'), in this case, all cells of a row are retrieved.
When setHorizontalAlignment is used in a loop, the process cost will become high.
When these points are reflected in your script, it becomes as follows.
Modified script:
function CenterAlignAllDashes() {
// Ref: https://stackoverflow.com/a/53678158
const columnIndexToLetter_ = index => (a = Math.floor(index / 26)) >= 0 ? columnIndexToLetter_(a - 1) + String.fromCharCode(65 + (index % 26)) : "";
var sheet = SpreadsheetApp.getActiveSheet();
var topRow = sheet.getRange(1, 1, 1, sheet.getLastColumn());
var topRowValues = topRow.getValues()[0];
var ranges = [];
for (let i = 0; i < topRowValues.length; i++) {
if (topRowValues[i] == '-') {
ranges.push(`${columnIndexToLetter_(i)}1`);
}
}
sheet.getRangeList(ranges).setHorizontalAlignment("center");
}
When this script is run, first, the values are retrieved from the 1st row and create the range list. And, the alignment is changed using the range list.
References:
getLastColumn()
getValues()
getRangeList(a1Notations)
setHorizontalAlignment(alignment) of Class RangeList

Answer 2

Try this:
function CenterAlignAllDashes() {
const sh = SpreadsheetApp.getActiveSheet();
sh.getRange(1, 1, 1, sh.getLastColumn()).getValues().flat().forEach((v, i) => {
if (v == "-") {
sh.getRange(1, i + 1).setHorizontalAlignment("center");
}
});
}