toUpperCase can be used to make a string or variable upper case. In this example, "Hello World" will become "HELLO WORLD".

const my_string = "Hello World";
console.log(my_string.toUpperCase());

lower can be used to make a string or variable lower case. In this example, "Hello World" will become "hello world".

const my_string = "Hello World";
console.log(my_string.toLowerCase());

Here is an example with an if statement.

const my_string = "Hello World";

console.log(my_string.toLowerCase());

if (my_string.toLowerCase().includes('hello')) {
  console.log("my_string includes hello");
} else {
  console.log("my_string does NOT include hello");
}