AI solution for JavaScript Coding Question on BFE.dev
139. implement _.partial()

Here's one possible implementation of partial() in JavaScript:

const partial = (fn, ...presetArgs) => {
  const placeholder = Symbol()
  return function partiallyApplied(...laterArgs) {
    const allArgs = presetArgs.map(arg => arg === partial.placeholder ? laterArgs.shift() : arg).concat(laterArgs)
    return fn.apply(this, allArgs)
  }
}
partial.placeholder = Symbol()

// test code
const func = (...args) => args
const func123 = partial(func, 1, 2, 3)
console.log(func123(4)) // should print [1, 2, 3, 4]

const _ = partial.placeholder
const func1_3 = partial(func, 1, _, 3)
console.log(func1_3(2, 4)) // should print [1, 2, 3, 4]

The partial() function takes a function fn and any number of presetArgs that are bound to the function's parameters. It returns a new function that takes some additional laterArgs, which are appended to the preset arguments to form a complete argument list for the original function call.

The key feature of this implementation is support for placeholders, which are stored in a special Symbol object that's attached to the partial() function as a property called placeholder. The partiallyApplied() function checks each of its presetArgs and, if it encounters a placeholder, it pops the next value from the laterArgs array and uses that value in place of the placeholder. If there are no more values left in laterArgs, the placeholder remains unchanged.

Note that we define partial.placeholder = Symbol() so that the test code can use _ as a shorthand for partial.placeholder. Alternatively, we could simply have used undefined or some other distinctive value as our placeholder symbol.