Demo of a hacky def function for a lambda-like syntax in javascript

def.js

// def(function() { $[0]+$[1] }) --> $ is a real array of args, adds a return
var def = function(f) {
    // Another option is like this so $0, $1 etc
    //return eval("("
    //            +(f.toString()
    //              .replace(/\$_/g, "arguments[0]")
    //              .replace(/\$[\d+]/g, function(r,num) { return "arguments["+num+"]" })
    //              .replace(/\) { /, ") { return ")
    //              )+")");
    // Hacks around for trailing ; issue, and suffers from closure issues (when we eval here
    // any local symbols have vanished) but the idea is good... see caterwauljs.org for the
    // real thing (https://github.com/spencertipping/caterwaul) !!
    // Inspiration from http://www.spencertipping.com

    // Note [\s\S] matches any char, whereas '.' won't match a \n
    return eval(("("+f+")").replace(/\) \{([\s\S]*)\}\)$/, function(s,m) {
        return (") {"
                +"var $ = Array.prototype.slice.call(arguments,0), $_ = $[0];"
                +"return ("+m.replace(/;\s+$/, "")+"); })");
    }));
};
var add2 = def(function() { $[0] + $[1] });
var sum  = def(function() { $_.reduce(add2, 0) });

Note that you can still name parameters if you want
add2 = def(function(x,y) { x+y; })
and you can avoid breaking closures if you don't mind moving the eval outside of def() (ie this is tailored to my personal taste, feel free to fiddle)