defn() Function wrapper

defn.js

/**
 * Define a function with an optional name
 * 
 * Primarily useful for defining named functions with additional configuration based on method calls on the defined function since
 * `const fn = (...) => {...}.fnMethod()` is invalid syntax, so you need to wrap the function definition with parens anyway.
 * 
 * ```js
 * const concat = defn('concat', (a, b) => a + b).withType([String, String], String);
 * ```
 * 
 * @param {string|Function} nameOrFnDef The function to define with no name or a value that will be stringified to the function name
 * @param {Function} [fnDef] The function definition if {@link nameOrFnDef} is provided
 * @returns {Function} A function with a name if both args are provided, or an anonymous function if only 1 arg is provided
 * @throws {TypeError} Thrown if a function was not provided to define
 */
const defn = (nameOrFnDef, fnDef) => {
    // Process the multiple possible signatures
    const [name, fn] = fnDef && typeof nameOrFnDef !== 'function'
        ?[String(nameOrFnDef), fnDef]
        : ['', nameOrFnDef];
    
    // We need a function to define and optionally name
    if (typeof fn !== 'function') {
        throw new TypeError('A function was not provided to `defn()` to define');
    }

    // The function already has a `.name` which means it's probably already been defined
    if (fn.name) {
        return fn;
    }

    return name ? Object.defineProperty(fn, 'name', { value: name, configurable: true }) : fn;
}