JavaScript Implementation of String.hashCode() .

string.hashcode.js

/**
 * Returns a hash code for a string.
 * (Compatible to Java's String.hashCode())
 *
 * The hash code for a string object is computed as
 *     s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
 * using number arithmetic, where s[i] is the i th character
 * of the given string, n is the length of the string,
 * and ^ indicates exponentiation.
 * (The hash value of the empty string is zero.)
 *
 * @param {string} s a string
 * @return {number} a hash code value for the given string.
 */
hashCode = function(s) {
  var h = 0, l = s.length, i = 0;
  if ( l > 0 )
    while (i < l)
      h = (h << 5) - h + s.charCodeAt(i++) | 0;
  return h;
};

output.txt

> hashCode('')
0
> hashCode('Hello')
69609650

@ngp33 Indeed, it's not a good hash function, but some people need compatibility with Java.
I posted this answer in direct response to this algorithm constantly being shared around, for those needing a rock-solid general function.

Just FYI, another take on the same problem here, with both an old Java-style hash and with a version of @bryc's excellent, more modern cyrb53 hash:
https://gist.github.com/jlevy/c246006675becc446360a798e2b2d781