How to convert snake case to camelcase in my app

Use _.mapKeys() instead of _.mapValues():

var data = {
   'id': '123',
   'employee_name': 'John',
   'employee_type': 'new'  
};

var newObj = _.mapKeys(data, (value, key) => _.camelCase(key));

console.log('newObj: ', newObj);
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If you need to ignore the redundant value param, you can use _.rearg() on _.camelCase() to generate a function that takes the 2nd param (the key) instead of the 1st param (the value).

var data = {
   'id': '123',
   'employee_name': 'John',
   'employee_type': 'new'  
};

var newObj = _.mapKeys(data, _.rearg(_.camelCase, 1));

console.log('newObj: ', newObj);
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replacing snake_case or kebab-case to camelCase only for string (ES6+):

const snakeToCamel = str =>
  str.toLowerCase().replace(/([-_][a-z])/g, group =>
    group
      .toUpperCase()
      .replace('-', '')
      .replace('_', '')
  );

result:

console.log(snakeToCamel('TO_CAMEL')) //toCamel
console.log(snakeToCamel('to_camel')) //toCamel
console.log(snakeToCamel('TO-CAMEL')) //toCamel
console.log(snakeToCamel('to-camel')) //toCamel

Here's how to do it in native Javascript...

let data = {
   'id':'123',
   'employee_name': 'John',
   'employee_type': 'new'  
}

// #1 simple function which converts a string from snake case to camel case ...
const snakeToCamel = s => s.replace(/(_\w)/g, k => k[1].toUpperCase())

// #2 create new data object with camelCase keys...
data = Object.entries(data).reduce((x,[k,v]) => (x[snakeToCamel(k)]=v) && x, {})

console.log(data)

You can also easily create your own function for that:

function camelCase(obj) {
  var newObj = {};
  for (d in obj) {
    if (obj.hasOwnProperty(d)) {
      newObj[d.replace(/(\_\w)/g, function(k) {
        return k[1].toUpperCase();
      })] = obj[d];
    }
  }
  return newObj;
}

var data = {
  'id': '123',
  'employee_name': 'John',
  'employee_type': 'new'
}

console.log(camelCase(data));