How to access the return statement of a function in Javascript

I'm looking for a way to get the return expression of a function in Javascript. I'm expecting to get it as a string but any type would do. Also, I'm looking for a way to set the return statement of a function without affecting the rest of it.

Example:

let f1 = () => {
    let x = 1
    return 2 + x
}

let f2 = () => {
    let y = 2
    return 3 + y
}

let s = f1.getReturnExpression()
console.log(s) // 2 + x

f2.setReturnExpression(s)
console.log(f2.toString()) // "function f2() {  let y = 2; return 2 + x; }