evalInScope.js

/*
 * Evaluate the expression `expr` in a scope that includes (a) the global scope; and
 * (b) the key-value pairs in `scope`.
 *
 *     evalInScope({a: 3, b: 4}, "a + b")  ===> 7
 *
 * It's up to the caller to make sure that `expr` really is a JS expression and
 * that it doesn't do anything unwanted; and that the keys of `scope` really are JS
 * identifiers. (That is, don't use this with a `scope` or `expr` you don't trust!)
 */
function evalInScope(scope, expr) {
    var keys = Object.keys(scope);
    var values = keys.map(function (key) { return scope[key]; });
    var f = Function(keys.join(", "), "return (" + expr + ");");

    // Note that at this point you could cache the function f.

    return f.apply(undefined, values);
}