This is for Reading a Single range

gapi.client.sheets.spreadsheets.values.get({
  spreadsheetId: spreadsheetId,
  range: range
}).then((response) => {
  var result = response.result;
  var numRows = result.values ? result.values.length : 0;
  console.log(`${numRows} rows retrieved.`);
});

This is for Reading Multiple ranges

var ranges = [
  // Range names ...
];
gapi.client.sheets.spreadsheets.values.batchGet({
   spreadsheetId: spreadsheetId,
   ranges: ranges
}).then((response) => {
  var result = response.result;
  console.log(`${result.valueRanges.length} ranges retrieved.`);
});

Read their Documentation : https://developers.google.com/sheets/api/guides/values#javascript