A quick test to get a recursive array flatten function in JS.

recursiveArrayFlatten.html

<script type="text/javascript">
	const processString = () => {
		try {
			const arrayString = document.forms["myForm"]["array"].value;
			const jsonDecodedArray = JSON.parse(arrayString);
			const resultArray = arrayFlatten(jsonDecodedArray);
			const resultArrayString = JSON.stringify(resultArray);
			document.getElementById("result").innerHTML = `Result: ${resultArrayString}`;
		}
		catch(error) {
			document.getElementById("result").innerHTML = `Error: ${error.message}`;
		}
		return false;
	};

	const arrayFlatten = unsortedArray => {
		let resultArray = []
		unsortedArray.forEach(element => {
			if (element instanceof Array) return resultArray = resultArray.concat(arrayFlatten(element));
			return resultArray.push(element);
		});
		return resultArray;
	};
</script>

<form name="myForm" onsubmit="return processString()" >
	<input name="array" />
	<input type="submit" />
</form>


<div id="result"></div>