Why Cannot read property 'length' of undefined error?

Question

JavaScript

an error in line 1.. it is impossible so whether to take the length of the line..or am I accessing the array element and do not get the line?

function findLongest(str) {
  
  var spl = str.split(" ");
  var longest = 0
  
  for (var i = 0; i <= spl.length; i++) {
    if (spl[i].length > longest) {    //(1)
      longest = spl[i].length;
    }
    }
    return longest;
}
console.log(findLongest("The quick white fox jumped around the massive dog"), 7);