A short helper to 'underscorify' strings and objects (taking nesting into account). It uses underscore (though vanilla js shouldn't be difficult) and ES6 syntax.

underscorify.js

import _ from 'underscore';

// Convert fooBar to foo_bar
const underscoreCase = function(str) {
  return str.replace(/([a-z])([A-Z]+)/g, function(terms) {
    return terms[0] + '_' + terms[1].toLowerCase();
  });
};

const underscorify = function(stringOrObject) {
  if (_.isString(stringOrObject)) {
    return underscoreCase(stringOrObject);
  }

  return Object.keys(object || {}).reduce(function(memo, key) {
    const value = object[key];
    const newValue = _.isObject(value) ? underscorifyObject(value) : value;
    memo[underscoreCase(key)] = newValue;
    return memo;
  }, {});
};

export default underscorify;


// specs for it
describe('underscorify', function() {
  it('should transform fooBar into foo_bar', function() {
    expect(underscorify('fooBar')).toEqual('foo_bar');
  });

  it('should transform { fooBar: \'foo\' } into { foo_bar: \'foo\' }', function() {
    expect(underscorify({ fooBar: 'bar' })).toEqual({ foo_bar: 'bar' });
  });

  it('should also transform nested objects', function() {
    expect(underscorify({
      fooBar: 'bar',
      nestedAttr: {
        fooBar: 'bar'
      }
    })).toEqual({
      foo_bar: 'bar',
      nested_attr: {
        foo_bar: 'bar'
      }
    });
  });
});