Try this:

• ✓ Works with alpha rgba(255, 255, 255, 0) => #ffffff00
• ✓ Works with single digits rgba(0, 0, 0, 0) => #00000000
• ✓ Works with RGB as well rgb(124, 255, 3) => #7cff03
• ✓ You can slice alpha channel away rgba(255, 255, 255, 0) => #ffffff

function RGBAToHexA(rgba, forceRemoveAlpha = false) {
  return "#" + rgba.replace(/^rgba?\(|\s+|\)$/g, '') // Get's rgba / rgb string values
    .split(',') // splits them at ","
    .filter((string, index) => !forceRemoveAlpha || index !== 3)
    .map(string => parseFloat(string)) // Converts them to numbers
    .map((number, index) => index === 3 ? Math.round(number * 255) : number) // Converts alpha to 255 number
    .map(number => number.toString(16)) // Converts numbers to hex
    .map(string => string.length === 1 ? "0" + string : string) // Adds 0 when length of one number is 1
    .join("") // Puts the array to togehter to a string
}

//
// Only tests below! Click "Run code snippet" to see results
//

// RGBA with Alpha value
expect(RGBAToHexA("rgba(255, 255, 255, 0)"), "#ffffff00")
expect(RGBAToHexA("rgba(0, 0, 0, 0)"), "#00000000")
expect(RGBAToHexA("rgba(124, 255, 3, 0.5)"), "#7cff0380")
expect(RGBAToHexA("rgba(124, 255, 3, 1)"), "#7cff03ff")

// RGB value 
expect(RGBAToHexA("rgba(255, 255, 255)"), "#ffffff")
expect(RGBAToHexA("rgba(0, 0, 0)"), "#000000")
expect(RGBAToHexA("rgba(124, 255, 3)"), "#7cff03")

// RGBA without Alpha value
expect(RGBAToHexA("rgba(255, 255, 255, 0)", true), "#ffffff")
expect(RGBAToHexA("rgba(0, 0, 0, 0)", true), "#000000")
expect(RGBAToHexA("rgba(124, 255, 3, 0.5)", true), "#7cff03")
expect(RGBAToHexA("rgba(124, 255, 3, 1)", true), "#7cff03")

function expect(result, expectation) {
  console.log(result === expectation ? "✓" : "X", result, expectation)
}

Code is built based on:

• https://css-tricks.com/converting-color-spaces-in-javascript/

Assuming that the values are 0...1 per channel. And assuming that the abbreviation in the method / function call in the question correspond to the arguments, the following should work.

A = 255 * 0.86
R = 255 * 0
G = 255 * 0
B = 255 * 0

Note you may want to change how it rounds off here as it may give inaccuracies in colours.

At this point, the values are in fact still floating point values, but casting them to a byte or a char (depending on language), should in theory work.

var _A = (byte)219.3
var _R = (byte)0
var _G = (byte)0
var _B = (byte)0

Now all you have to do is convert them to a hexadecimal string each, and concatenate them (ARGB) and put a nice little hash tag in front (#)

In C# you could do something akin to:

var hexString = string.Format("#{0:X2}{1:X2}{2:X2}{3:X2}", _A, _R, _G, _B);

Yielding a final result of something like:

#DB000000

function hexify(color) {
  var values = color
    .replace(/rgba?\(/, '')
    .replace(/\)/, '')
    .replace(/[\s+]/g, '')
    .split(',');
  var a = parseFloat(values[3] || 1),
      r = Math.floor(a * parseInt(values[0]) + (1 - a) * 255),
      g = Math.floor(a * parseInt(values[1]) + (1 - a) * 255),
      b = Math.floor(a * parseInt(values[2]) + (1 - a) * 255);
  return "#" +
    ("0" + r.toString(16)).slice(-2) +
    ("0" + g.toString(16)).slice(-2) +
    ("0" + b.toString(16)).slice(-2);
}

var myHex = hexify('rgba(57,156,29,0.05)'); // "#f5faf3"

console.log(myHex);