How to write a function that only invokes a callback once, then returns the result of calling the function the first time, every subsequent calling

Question

How to write a function that returns a callback, but only the result of the first time calling the callback.

Returns a function that is restricted to invoking func once. Repeat calls to the function return the value of the first call.

function once(func) {
 let result = func
 let counter = 0
 //let result 
 function inner (n) {
   counter++
   if (counter = 1) {
     return func
    }
   if (counter > 1) {
     return result
   }
 }
 return inner
}

I can't put my finger on what I'm doing wrong.

It's not really clear what you are trying to do. Your demo doesn't call `once` at all. In the function you cope `func` to `result` and then never change either so you just have to variables with the same value. You've go an assignment in the middle of an if where I'd expect a comparison. — Quentin, Mar 03 '22 at 17:03

score 0 · Answer 1 · answered Mar 03 '22 at 17:10

You're on the right track using the closure, you just have to

Call func at least once (right now you just refer to it; more here)
Remember the result of that for subsequent invocations

You don't need a counter, just some kind of flag; perhaps func itself, which you could clear:

function once(func) {
    let result;
    return () => {
        if (func) {
            result = func();
            func = null;
        }
        return result;
    };
}

function once(func) {
    let result;
    return () => {
        if (func) {
            result = func();
            func = null;
        }
        return result;
    };
}

const fn = () => {
    console.log("fn called");
    return 42;
};

console.log("Getting the function");
const f = once(fn);
console.log("Doing first call");
console.log(f());
// => fn called
// => 42
console.log("Doing second call");
console.log(f()); // 42
// => 42
console.log("Doing third call");
console.log(f()); // 42
// => 42

How to write a function that only invokes a callback once, then returns the result of calling the function the first time, every subsequent calling

1 Answers1