string expression = "2+2";

public string evaluateExpresion(expression)
{
   return executeJavascript(expression); // Magic javascript executor
}

Debug.Log(evaluateExpression(expression);

// JavaScriptExpressions.js

function executeExpression( x : string ) {
    // Some code here..
}

// YourCsharp.cs
string expression = "2+2";

public string evaluateExpresion(string expression) {
    return transform.GetComponent<JavaScriptExpressions>().executeExpression(expression);
}

Debug.Log(evaluateExpression(expression));