getFnParams: takes a function and returns the names of that function's parameters as an array

get-fn-params.js

const fnParamsRe = /^(?:async)?\s*(?:(?:function\s+[a-zA-Z_][a-zA-Z0-9_]*|function|[a-zA-Z_][a-zA-Z0-9_]*)\s*(?:\(([^)]*)\))|(?:\(([^)]*)\)|([a-zA-Z_][a-zA-Z0-9_]*))\s*=>\s*{)/
const sepRe = /\s*,\s*/g
const getFnParams = fn => {
  const source = fn.toString()
  const match = fnParamsRe.exec(source) || []
  const paramStr = match[1] || match[2] || match[3] || ''
  return paramStr.split(sepRe).filter(Boolean)
}


(() => {
  // function
  function f1(param) { }
  function f2(p1, p2) { }
  async function f3(p1, p2) { }

  // arrow
  const a1 = param => { }
  const a2 = (p1, p2) => { }
  const a3 = async param => { }
  const a4 = async (p1, p2) => { }

  // member
  const obj = {
    m1(p1, p2) { },
    async m2(p1, p2) { },
    m3: function (p1, p2) { },
    m4: async function (p1, p2) { },
    m5: param => { },
    m6: async param => { },
    m7: (p1, p2) => { },
    m8: async (p1, p2) => { },
  }
  const testFns = [
    // function
    f1, f2, f3, function (p1, p2) { },

    // arrow
    a1, a2, a3, a4, param => { }, (p1, p2) => { },

    // member
    obj.m1, obj.m2, obj.m3, obj.m4, obj.m5, obj.m6, obj.m7
  ]
  const testParams = [
    // function
    ['param'],
    ['p1', 'p2'],
    ['p1', 'p2'],
    ['p1', 'p2'],

    // arrow
    ['param'],
    ['p1', 'p2'],
    ['param'],
    ['p1', 'p2'],
    ['param'],
    ['p1', 'p2'],

    // member
    ['p1', 'p2'],
    ['p1', 'p2'],
    ['p1', 'p2'],
    ['p1', 'p2'],
    ['param'],
    ['param'],
    ['p1', 'p2'],
    ['p1', 'p2']
  ]

  const arrsAreEqual = (a1, a2) => {
    if (a1.length !== a2.length) return false
    for (let i = 0, l = a1.length; i < l; i++) {
      if (a1[i] !== a2[i]) return false
    }
    return true
  }


  let failCount = 0
  for (let i = 0, l = testFns.length; i < l; i++) {
    const testFn = testFns[i]
    const expected = testParams[i]
    const actual = getFnParams(testFn)
    const passed = arrsAreEqual(expected, actual)
    if (!passed) {
      failCount++
      console.log(`  #${i.toString().padStart(8)}failed: ${testFn.toString()}`)
    }
  }

  if (failCount) {
    const rate = Math.round(failCount / testFns.length * 100)
    const red = '\x1b[31m'
    const reset = '\x1b[0m'
    console.log(`${red}\n${failCount} failed tests (${rate}%).${reset}`)
  } else {
    console.log(`\nAll tests passed.`)
  }
})()