4. What is apply function?

apply invoked on a function allows to pass an object, which will be used as this. It loads context of the function

apply is different from bind - in apply function is invoked immediately with parameters passed as a second argument of apply. This second argument is an array of parametres, which will be passed to function in the same order.

const person = {
  firstName: "Mario",
  print: function (arg1, arg2) {
    console.log(
      `I'm ${this?.firstName} and this is an arg1: ${arg1} and arg2: ${arg2}`
    );
  },
};

person.print(); // I'm Mario and this is an arg1: undefined and arg2: undefined

const printFunction = person.print;
printFunction(); // I'm undefined and this is an arg1: undefined and arg2: undefined

printFunction.apply(person); // I'm Mario and this is an arg1: undefined and arg2: undefined

printFunction.apply(person, ["arg1", "arg2"]); // I'm Mario and this is an arg1: arg1 and arg2: arg2

apply is different from call - in apply arguments passed to function are taken from array passed as second argument, while in call they are speparated with comma.

printFunction.apply(person, ["arg1", "arg2"]); // I'm Mario and this is an arg1: arg1 and arg2: arg2
printFunction.call(person, "arg1", "arg2"); // I'm Mario and this is an arg1: arg1 and arg2: arg2