Algorithm

• Create a Object for each symbol as key and it's respective
  value
  const romanobj = {
I : 1,
V : 5,
X : 10,
L : 50,
C : 100,
D : 500,
M : 1000
}

• Split the Roman Numeral string into Roman Symbols or in different word split string into Characters.

• Take a Symbol from Starting from 0 index
  1). If current value of symbol is greater than or equal to
  the value of next symbol, then add this value to the
  total.
  2). else subtract this value by adding the value of next
  symbol to the total.

Javascript Function to Covert Roman Numeral to Integer

/**
 * @param {string} s
 * @return {number}
 */
const obj = {
I      :     1,
V      :     5,
X      :     10,
L      :     50,
C      :     100,
D      :     500,
M      :     1000
}
var romanToInt = function(s) {
let result = 0;
for(let i=0;i<s.length;i++){
if(i+1<s.length){
if(obj[s[i]]>=obj[s[i+1]]){
 result = result + obj[s[i]];

}else{
 result = result + obj[s[i+1]]-obj[s[i]];
i++;
 } 
}else{
result = result + obj[s[i]];
}
}
return result;

};

Test Cases

Input: s = "III"
Output: 3
Explanation: III = 3.

Input: s = "MCMXCIV"
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.