As mentioned in the comments, sure you can, and it even makes sense as long as you are checking for different conditions.

However, this reads much more natural:

if (condition1) {
  //  Your code running if condition1 === true
} else if (condition2) {
  //  Some other code running if condition2 === true
} else {
  //  Code running for all other cases
}

Always avoid nesting if possible.

Also, there is no point in checking if a condition === true, as in itself it represents a boolean value.

There is also something to be said for never using else at all... But you can google and read about that to your own leisure.

Simply use an encapsulation pattern. Extract the logic elsewhere, so your main function remains easy to read.

function checkFooEdgeCase(condition){
  return condition === "foo" ? "do foo stuff" : "do other stuff" 

function foo(condition) {
  if (condition === "true") return "do stuff" 
  return checkFooEdgeCase(condition) 
}

Yes, you can. consider the example below:

let sum = 35;
if (sum % 10 == 0){
    console.log("a");
} else if (sum % 2 == 1){
    if (sum % 5 == 0 && sum % 2 == 0){
        console.log("b");
    } else if (sum % 5 == 0){
        console.log("c")
    } else {
        console.log("d")
    }
} else {
    console.log("e")
}

More examples and explanation here: https://exlskills.com/learn-en/courses/javascript-fundamentals-basics_javascript/conditional-statements-zgrXFcSqdfIF/the-if-statements-YcHrGQKxvTOI/nested-if-statements-nSAoxbDFvMOq