Use replace method in js to complete the conversion of a certain character

The code copy is as follows:

var str = "National [1990] No. 12";

How to prefix "[" and "]" in variables with backslashes,

It is to be replaced with this: str = "National name/[1990/] No. 12";

The code copy is as follows:

<script language="javascript" type="text/javascript">

var str = "National [1990] No. 12";

str=str.replace(//[/g,'//[');

str=str.replace(//]/g,'//]');

document.write(str);

</script>