GET功能控制台错误

Question

The logic:

my_function looks through file.txt, if it finds the word "Hello", returns true. If not, false.

The problem:

Uncaught type error: contents.includes is not a function

Limitations:

Can only use plain javascript

function my_function() {
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function(contents) {
if (this.readyState == 4 && this.status == 200) {
//contents variable now contains the contents of the textfile as string

//check if text file contains the word Hello
var hasString = contents.includes("Hello");

//outputs true if contained, else false
console.log(hasString);

}
};
xhttp.open("GET",  "http://www.example.com/file.txt", true);
xhttp.send();
}