According to this esdiscuss discussion, it is possible in ECMAScript 6 to define multiline strings without having to place subsequent lines of the string at the very beginni
You can also just do a string replace for double spaces (assuming your indentation uses spaces, not tabs). Obviously any double spaces in your actual string would be removed, but for most cases this should be ok.
const MSG = (`Line 1
line 2
line 3`).replace(/ +/g, '');
// outputs
/*
Line 1
line 2
line 3
*/