Async Function Composition

Function composition is a powerful programming technique where you combine multiple functions to create a new function. This challenge focuses on implementing a robust function composition mechanism that seamlessly handles asynchronous JavaScript functions (those returning Promises). This is crucial for building complex, sequential asynchronous operations in a readable and maintainable way.

Problem Description

Your task is to implement a function named composeAsync that takes an arbitrary number of asynchronous functions as arguments. composeAsync should return a new asynchronous function. When this returned function is called, it should execute the provided asynchronous functions in sequence, from right to left, passing the result of each function as the argument to the next. The final result of the composed function should be the result of the last function in the sequence.

Key Requirements:

1. Handle Asynchronous Functions: The input functions can be synchronous or asynchronous (returning Promises). The composeAsync function must correctly handle the resolution of Promises.
2. Right-to-Left Execution: Functions should be executed in reverse order of how they are passed to composeAsync.
3. Argument Passing: The output of one function becomes the input of the next. The initial value passed to the composed function will be the input to the rightmost function.
4. Return a Promise: The composed function itself must always return a Promise, regardless of whether the individual functions are synchronous or asynchronous.
5. Error Handling: If any function in the composition chain rejects its Promise (or throws an error synchronously), the composed function should immediately reject with that error.

Expected Behavior:

If composeAsync(f, g, h) is called and returns composedFn, then composedFn(initialValue) should behave like f(g(h(initialValue))), where all intermediate results are properly awaited if they are Promises.

Edge Cases:

• No functions provided: If composeAsync is called with no arguments, it should return a function that simply returns its input.
• Single function provided: If composeAsync is called with a single function, it should return that function (wrapped to always return a Promise).
• Synchronous and Asynchronous Mix: The composition should work seamlessly with a mix of synchronous and asynchronous functions.

Examples

Example 1:

const asyncAdd1 = async (x) => {
  await new Promise(resolve => setTimeout(resolve, 50));
  return x + 1;
};

const asyncMultiply2 = async (x) => {
  await new Promise(resolve => setTimeout(resolve, 50));
  return x * 2;
};

const asyncSubtract3 = async (x) => {
  await new Promise(resolve => setTimeout(resolve, 50));
  return x - 3;
};

const composed = composeAsync(asyncSubtract3, asyncMultiply2, asyncAdd1);

// Call the composed function
composed(5).then(result => {
  console.log(result); // Expected: 9
});

Explanation:

1. asyncAdd1(5) returns Promise<6>.
2. asyncMultiply2(6) returns Promise<12>.
3. asyncSubtract3(12) returns Promise<9>. The final result is 9.

Example 2:

const syncSquare = (x) => x * x;
const asyncDouble = async (x) => x * 2;
const syncAdd10 = (x) => x + 10;

const composed = composeAsync(syncAdd10, asyncDouble, syncSquare);

// Call the composed function
composed(3).then(result => {
  console.log(result); // Expected: 28
});

Explanation:

1. syncSquare(3) returns 9.
2. asyncDouble(9) returns Promise<18>.
3. syncAdd10(18) returns 28. The final result is 28.

Example 3: Error Handling

const asyncSuccess = async (x) => x + 1;
const asyncFailure = async (x) => {
  throw new Error("Something went wrong!");
};
const syncAdd2 = (x) => x + 2;

const composed = composeAsync(syncAdd2, asyncFailure, asyncSuccess);

// Call the composed function
composed(10).catch(error => {
  console.error(error.message); // Expected: "Something went wrong!"
});

Explanation:

1. asyncSuccess(10) returns Promise<11>.
2. asyncFailure(11) throws an error.
3. The composed function immediately rejects with the error from asyncFailure. syncAdd2 is never called.

Example 4: No Functions

const composedNoArgs = composeAsync();

composedNoArgs(100).then(result => {
  console.log(result); // Expected: 100
});

Explanation: When no functions are provided, composeAsync should return a function that resolves with the input value.

Constraints

• The number of functions passed to composeAsync can range from 0 to any reasonable number (e.g., 100).
• Input values to the composed function can be any JavaScript type that can be passed through functions.
• The performance of the composition mechanism itself should be efficient. The overhead of composeAsync should be minimal, and the primary performance bottleneck should be the execution of the individual functions.

Notes

• Consider how you will handle both synchronous and asynchronous operations within the composition. Promise.resolve() can be a useful tool.
• Think about the iteration strategy for applying functions from right to left.
• The reduceRight array method might be particularly helpful here.
• Ensure that errors thrown synchronously by a function also lead to the composed function rejecting.