You could try the below steps.

• Capture all the uppercase letters and also match the preceding optional dot character.

• Then convert the captured uppercase letters to lowercase and then return back to replace function with an _ as preceding character. This will be achieved by using anonymous function in the replacement part.

• This would replace the starting uppercase letter to _ + lowercase_letter.

• Finally removing the starting underscore will give you the desired output.

  var s = 'TypeOfData.AlphaBeta';
  console.log(s.replace(/(?:^|\.?)([A-Z])/g, function (x,y){return "_" + y.toLowerCase()}).replace(/^_/, ""));
  

OR

var s = 'TypeOfData.AlphaBeta';
alert(s.replace(/\.?([A-Z])/g, function (x,y){return "_" + y.toLowerCase()}).replace(/^_/, ""));

any way to stop it for when a whole word is in uppercase. eg. MotorRPM into motor_rpm instead of motor_r_p_m? or BatteryAAA into battery_aaa instead of battery_a_a_a?

var s = 'MotorRMP';
alert(s.replace(/\.?([A-Z]+)/g, function (x,y){return "_" + y.toLowerCase()}).replace(/^_/, ""));

str.split(/(?=[A-Z])/).join('_').toLowerCase();

u're welcome

var s1 = 'someTextHere';
var s2 = 'SomeTextHere';

var o1 = s1.split(/(?=[A-Z])/).join('_').toLowerCase();
var o2 = s2.split(/(?=[A-Z])/).join('_').toLowerCase();

console.log(o1);
console.log(o2);

Alternatively using lodash:

lodash.snakeCase(str);

Example:

_.snakeCase('TypeOfData.AlphaBeta');
// ➜ 'type_of_data_alpha_beta'

Lodash is a fine library to give shortcut to many everyday js tasks.There are many other similar string manipulation functions such as camelCase, kebabCase etc.