flattenArrayExample.js

/**
 * Function to flatten an array of arbitrarily nested arrays of integers into a flat array of integers
 *
 * @param {Array}  inputArray    The array to be flattenned.
 * @param {Array}  resultArray   The result array.
 */
flattenArray = (inputArray, resultArray) => {
	inputArray.forEach(function(entry) {
  	if(entry.constructor === Array){
    	flattenArray(entry, resultArray);
    } else {
    	resultArray.push(entry);
    }
	});
}

// Example usage
const a = [[1,2,[3]],4];
const rslt = [];
flattenArray(a, rslt);
console.log(rslt);