I have a piece of JavaScript that is supposed to update a in my HTML:
var StringContent = ({
    \"a\": \'Some String a\',
    \"b\": \'Some string b\',
            
        
You should replace \n by  since innerHTML takes an HTML string (in HTML, \n merges with adjacent spaces but does not produce a carriage return except for elements with the style white-space:pre-wrap as noted by MaxArt) :
document.getElementById("overlaycontent").innerHTML = (
    StringContent.a + '
' +
    StringContent.b + '
' +
    StringContent.c,
)