Write classNames(...args) to build a CSS class string from strings, objects, and arrays.
Strings and numbers are added as they are. For an object, each key is added when its value is truthy. Arrays are handled recursively. Falsy arguments (false, null, undefined, 0, "") and true are ignored. Join the classes with single spaces.
function classNames(...args) {
// your code
}
// classNames("btn", { active: true, disabled: false }, ["lg", null]); // "btn active lg"
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