Error message regarding array index

You are trying to iterate to i <= 15 and getting error when JavaScript evaluate Line[1] because it is undefined

Instead you can iterate til the length of Line array:

var Line = [{ Func: "Function"}];
for (var i = 0, len = Line.length; i < len; i++) {
  Line[i].Func = "Function" + i;
}

console.log(Line);

Or iterate using Array.prototype.forEach():

var Line = [{ Func: "Function"}];
Line.forEach((el, index) => el.Func = "Function" + index);

console.log(Line);

Or do your way but first check the element Line[i] exist:

var Line = [{ Func: "Function"}];
for (var i = 0; i <= 15; i++) {
  Line[i] && (Line[i].Func = "Function" + i);
  console.log(i);
}

console.log(Line);

Because you have only one element in the array.

var Line = [{ Func: "Function"}];
for (i = 0; i <= 15; i++) { console.log(i); Line[i].Func = "Function" + i;}

will print 0 and 1 and then it will throw an error

You are looping from 0 through to 15, and using the loop variable as an index into an array that only has 1 element (index=0)

Looks like you're trying to add new elements

var Line = [{ Func: "Function"}];
for (i = 0; i <= 15; i++) {
    Line[i] = {Func:"Function" + i}; 
}

You have a one-length array and you're iterating up to 15. So, you can either check the array limit, or provide a default value up to 15.

This is how you can provide a default value up to 15:

var Line = [{ Func: "Function"}];
for (var i = 0; i <= 15; i++) {
  if (Line[i] === undefined) {
      Line[i] = { Func: "Function"}
  }
  Line[i].Func = "Function" + i;
}

On the other hand, this is how you can go through the actual list, regardless of the element number:

var Line = [{ Func: "Function"}];
for (var i = 0; i <= Line.length; i++) {
  Line[i].Func = "Function" + i;
}

Your Line array has only 1 object inside it and you are looping from 0 to 15, so after Line[0] it cannot find Line[1] or Line[2]. You probably want to add an object 15 times to Line. Try something like this within your for loop:

Line.push({Func:"Function" + i});

https://www.w3schools.com/jsref/jsref_push.asp