How get the true or false from function?

function mutation(arr) {
let arr1 = arr[0].toLowerCase().split("");

let arr2 = arr[1].toLowerCase().split("");



for (let i = 0; i < arr[0].length; i++){
  if (arr1.indexOf(arr2[i]) == -1){
    console.log("not found");
  
  } else { console.log(" found"); }

}
}

mutation(["hello", "hey"]);

link : https://learn.freecodecamp.org/javascript-algorithms-and-data-structures/basic-algorithm-scripting/mutations/

How return the Boolean from function to pass the quiz ?

Returning true or false would work.

You can have more than 1 return statement in a function, actually you can have as many return statements as you want/need. But keep in mind that the first to get executed will break the function and return the assigned value.
In your case you can define a return statement returning a certain Boolean value in the if statement to be executed when the Boolean expression associated with it true and a return statement returning the opposite value in the else statement.

if(true){
//your code
return true
} else {
//your code
return false;
}

You can also solve this with the ternary operator