decamelize 
Convert a camelized string into a lowercased one with a custom separator
Example:unicornRainbow→unicorn_rainbow
Install
$ npm install --save decamelize
Usage
const decamelize = require('decamelize');
decamelize('unicornRainbow');
//=> 'unicorn_rainbow'
decamelize('unicornRainbow', '-');
//=> 'unicorn-rainbow'
API
decamelize(input, [separator])
input
Type: string
separator
Type: string
Default: _
Related
See camelcase for the inverse.
License
MIT © Sindre Sorhus