Roman to Integer

A Roman numeral writes a number as a run of letters — I, V, X, L, C, D, M — each worth a fixed amount, read left to right and added up. The one twist: when a smaller-valued letter sits directly before a larger one, it's subtracted instead. So VI is 6 (5 + 1) but IV is 4 (5 − 1). You've seen these on clock faces, movie copyright dates, and Super Bowl logos.

Implement romanToInteger(s) that takes a valid Roman numeral string and returns its integer value.

Signature

function romanToInteger(s) {
  // s is a valid Roman numeral (1–3999)
  // returns the integer it represents
}

The seven symbols and their values:

I = 1    V = 5    X = 10    L = 50
C = 100  D = 500  M = 1000

Examples

romanToInteger('III');      // 3    (1 + 1 + 1)
romanToInteger('IX');       // 9    (10 − 1)
romanToInteger('LVIII');    // 58   (50 + 5 + 1 + 1 + 1)
romanToInteger('MCMXCIV');  // 1994 (1000 + 900 + 90 + 4)

Notes

• Input is always valid — a well-formed numeral in the range 1–3999. You don't need to validate it or handle empty strings.
• Only six subtractive pairs exist: IV (4), IX (9), XL (40), XC (90), CD (400), CM (900). Every subtraction is a single small symbol before a single larger one.
• A single left-to-right pass is enough. You never need to look back, only at the current symbol and the one after it.