eq.js

/**

# eq.js: minimalistic equation evaluator.

   eq(str, vars)

Features:
  - evaluate variables in the `vars` object
  - `str` can use any `Math` function or property: round, ceil, max, E

Examples:

    eq('round(test / 2)', { test: 5 })   // 3
    eq('floor(test / 2)', { test: 5 })   // 2
    eq('2 + 2')                          // 4
    eq('cos(PI)')                        // -1

Handy amongst other things for CLI node programs that accept numbers.

**/

(function (g) {
  function eq (input, vars) {
    try {
      return eval('with(Math){with(vars || {}){' + input + '}}');
    } catch (e) {
      return NaN;
    }
  };

  g.top ? g.eq = eq : module.exports = eq;
})(this);

Since security isn't a concern, why not just remove the requirement for the $ and use two withs? eq('round(x / 2)', { x:5 }) seems more natural to me.

Good point, at first I was concerned about collisions but there aren't that many props in Math:

Math.E                     Math.LN10                  Math.LN2                   Math.LOG10E                Math.LOG2E                 Math.PI                    Math.SQRT1_2               Math.SQRT2                 Math.abs                   Math.acos                  Math.asin                  Math.atan                  Math.atan2
Math.ceil                  Math.cos                   Math.exp                   Math.floor                 Math.log                   Math.max                   Math.min                   Math.pow                   Math.random                Math.round                 Math.sin                   Math.sqrt                  Math.tan