test.js

async function replaceSubTextParts(startsWithText) {
  TXTextControl.subTextParts.getCount(async function(count) {
    for (var i = count - 1; i >= 0; i--) {
      await replaceText(i, startsWithText);
    }
  })
}

function replaceText(i, startsWithText) {
  return new Promise(resolve => {
    TXTextControl.subTextParts.elementAt(i, function(stp) {
      stp.getName(function(name) {
        if (name.startsWith(startsWithText)) {
          stp.getStart(function(start) {
            stp.getLength(function(length) {
              TXTextControl.select(start - 1, length - 1, function() {
                TXTextControl.selection.setText("Replaced SubTextPart Text");
                TXTextControl.focus();
                TXTextControl.clear();
                resolve(true);
              })
            })
          })
        } else resolve(false);
      })
    })
  });
}