Javascript issue: how to string replace double quotes with a backslash and a double quote

Question

I want to convert string '"hello":1' to '\"hello\":1'

Notice the single backslash in the output above? But none of the below snippets yields this result. It is either 2 backslashes or none at all

 var str = '"hello":1';
 str.replace(/"/g, '\"') => '"hello":1'
 str.replace(/"/g, '\\"') => '\\"hello\\":1'
 str.replace(/"/g, '\\\"') => '\\"hello\\":1'