.includes() not working in Internet Explorer

String.prototype.includes is, as you write, not supported in Internet Explorer (or Opera).

Instead you can use String.prototype.indexOf. #indexOf returns the index of the first character of the substring if it is in the string, otherwise it returns -1. (Much like the Array equivalent)

var myString = 'this is my string';
myString.indexOf('string');
// -> 11

myString.indexOf('hello');
// -> -1

MDN has a polyfill for includes using indexOf: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/includes#Polyfill

EDIT: Opera supports includes as of version 28.

EDIT 2: Current versions of Edge supports the method. (as of 2019)

Or just put this in a Javascript file and have a good day :)

String.prototype.includes = function (str) {
  var returnValue = false;

  if (this.indexOf(str) !== -1) {
    returnValue = true;
  }

  return returnValue;
}

includes() is not supported by most browsers. Your options are either to use

-polyfill from MDN https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/includes

or to use

-indexof()

var str = "abcde";
var n = str.indexOf("cd");

Which gives you n=2

This is widely supported.

Problem:

Try running below(without solution) from Internet Explorer and see the result.

console.log("abcde".includes("cd"));

Solution:

Now run below solution and check the result

if (!String.prototype.includes) {//To check browser supports or not
  String.prototype.includes = function (str) {//If not supported, then define the method
    return this.indexOf(str) !== -1;
  }
}
console.log("abcde".includes("cd"));