What is the difference between the two ways of defining a function?

The main difference is the this context.

Method of defining a function	Difference
function declaration	The this context is determined by how the function was called. It is hoisted before execution.
arrow function	The this context is inherited from the surrounding (lexical) context. It is not hoisted (no hoisting).

// function declaration
function regularFunction() {
  console.log(this); // `this` depends on the call
}

// arrow function
const arrowFunction = () => {
  console.log(this); // `this` is inherited from the context where the function is defined
};

Arrow functions also do not have their own arguments object (it is taken from the outer context) and cannot be used as constructors with the new operator.