For Loop is not redefining what’s supposed to be defined

I have this for loop in which I’m trying to replace an index in underlineArr with a character and whenever I try to redefine it so It saves the current variable so I’m able to access it and change it later, nothing is being saved, It keeps using the default array while I’m trying to get the newly edited array.

For Loop:

       const content = collected.content.toLowerCase();
           for (let i = 0; i < underlineArr.length; i++) {
               msg = underlineArr[i] = word[i] == content && underlineArr[i] == "_" ? content : "_";
           }
           msg = underlineArr.join(" ");
           console.log(msg);

This is the underlineArr constant: https://i.stack.imgur.com/vtzj2.png

This is the word constant: "French Toast", Please note the constant is random from an array I created.

Lastly, collected.content is just a string returned.

The output: Output

Expected output:

_ r _ _ _ _ _ _ _ _ _ _
_ r e _ _ _ _ _ _ _ _ _
_ r e n _ _ _ _ _ _ _ _
_ r e n c _ _ _ _ _ _ _
.....