Generate a class name for a DOM element with a string, array, or object literal

create-class.js

// Courtesy of: https://github.com/jorgebucaran/hyperapp

function createClass(value) {
    if (typeof value === 'string') {
        return value;
    }
    let output = '';
    if (Array.isArray(value)) {
        for (let i = 0, len = value.length, tmp; i < len; i++) {
            if ((tmp = createClass(value[i])) !== '') {
                output += (output && ' ') + tmp;
            }
        }
    } else {
        for (const cls in value) {
            if (value[cls]) {
                output += (output && ' ') + cls;
            }
        }
    }
    return output;
}

// Usage:

createClass('foo'); //=> "foo"
createClass(['foo', 'bar']); //=> "foo bar"
createClass({foo: true, bar: false, baz: true}); //=> "foo baz"