.NET's String.GetHashCode in C# and JavaScript

_GetHashCode.md

This is an equivalent implementation of .NET's String.GetHashCode in "safe" C# and ES5 JavaScript. If you write your own version in another language, feel free to comment below!

Notes

• The implementation (and therefore the generated hashcode) varies depending on the version of .NET that the app was compiled with. Here I provide .NET 3.5 and .NET 4.0, although it's likely that other versions work differently.
• In .NET 4.0, GetHashCode will often overflow the 32-bit signed integer limit. Other languages often use double-precision floating point values to represent all numbers, and will need to implement 32-bit signed integer overflow to mimic this.
• Be mindful of accuracy when multiplying large numbers. For example in C# 782507206 + 1369868898 * 1566083941 is 2145329683215674224, but in JavaScript it's represented as 2145329683215674400.

Tests

Input	.NET 3.5	.NET 4.0
"foo"	101574	1502598398
"foobar"	-1268878963	836807663
"the quick brown fox jumped over the lazy dog"	-772038699	-1284118495

GetHashCode.cs

// .NET 3.5 implementation
public int GetHashCodeDotNet35(string str)
{
    int hashCode = 0;
    foreach (char ch in str)
    {
        hashCode = (hashCode << 5) - hashCode + (int)ch;
    }
    return hashCode;
}

// .NET 4.0 implementation
public static int GetHashCodeDotNet40(string str)
{
    int hash1 = 5381;
    int hash2 = hash1;

    for (int i = 0; i < str.Length; i += 2)
    {
        char currentChar = str[i];
        hash1 = (hash1 << 5) + hash1 ^ currentChar;

        if (i + 1 < str.Length)
        {
            char nextChar = str[i + 1];
            hash2 = (hash2 << 5) + hash2 ^ nextChar;
        }
    }
    return hash1 + hash2 * 1566083941;
}

GetHashCode.js

function getHashCodeDotNet35(str)
{
    var hash = 0;
    for (var i = 0; i < str.length; i++)
    {
        var charCode = str.charCodeAt(i);
        hash = (hash << 5) - hash + charCode;
    }
    
    // Force conversion to 32-bit signed integer
    hash = hash & hash; 
    
    return hash;
}

function getHashCodeDotNet40(str)
{
    var hash1 = 5381;
    var hash2 = hash1;

    for (var i = 0; i < str.length; i += 2)
    {
        var currentChar = str.charCodeAt(i);
        hash1 = (hash1 << 5) + hash1 ^ currentChar;

        if (i + 1 < str.length)
        {
            var nextChar = str.charCodeAt(i + 1);
            hash2 = (hash2 << 5) + hash2 ^ nextChar;
        }
    }

    // Use BigInt because JavaScript loses accuracy with very large regular floating point numbers
    var result = BigInt(hash1) + BigInt(hash2) * BigInt(1566083941);

    // Convert the BigInt to a signed 32-bit integer (performs overflow)
    return Number(BigInt.asIntN(32, result));
}