Q.

Description:
The rgb function is incomplete. Complete it so that passing in RGB decimal values will result in a hexadecimal representation being returned. Valid decimal values for RGB are 0 - 255. Any values that fall out of that range must be rounded to the closest valid value.
Note: Your answer should always be 6 characters long, the shorthand with 3 will not work here.
The following are examples of expected output values:
rgb(255, 255, 255)//returns FFFFFF
rgb(255, 255, 300)//returns FFFFFF
rgb(0,0,0)//returns 000000
rgb(148, 0, 211)//returns 9400D3

A)

function rgb(r, g, b){
  // complete this function  
 const convert = (color) => {
   if (color.toString(16).length === 1 ) return '0' + color.toString(16).toUpperCase()
   if (color < 0) return '00'
   if (color >= 255 ) return 'FF'
   return color.toString(16).toUpperCase()
   }
 return convert(r) + convert(g) + convert(b)
}

補助関数では、最初のif文は、戻り文にsliceを与えることで除去できます.
1桁の面00、2つとも持って行き、2桁の面000、後ろは2つの位置しか持って行かない.

function rgb(r, g, b){
  // complete this function  
 const convert = (color) => {
   //if (color.toString(16).length === 1 ) return '0' + color.toString(16).toUpperCase()
   if (color < 0) return '00'
   if (color >= 255 ) return 'FF'
   return ('0' + color.toString(16).toUpperCase()).slice(-2)
   }
 return convert(r) + convert(g) + convert(b)
}