Solves advent of code day 7. To get the second answer copy the value of wire a into the assign command for b and run again.

aoc-7.js

var wires = {},
commands = [ 'bn RSHIFT 2 -> bo', 'lf RSHIFT 1 -> ly', 'fo RSHIFT 3 -> fq', 'cj OR cp -> cq', 'fo OR fz -> ga', 't OR s -> u', 'lx -> a', 'NOT ax -> ay', 'he RSHIFT 2 -> hf', 'lf OR lq -> lr', 'lr AND lt -> lu', 'dy OR ej -> ek', '1 AND cx -> cy', 'hb LSHIFT 1 -> hv', '1 AND bh -> bi', 'ih AND ij -> ik', 'c LSHIFT 1 -> t', 'ea AND eb -> ed', 'km OR kn -> ko', 'NOT bw -> bx', 'ci OR ct -> cu', 'NOT p -> q', 'lw OR lv -> lx', 'NOT lo -> lp', 'fp OR fv -> fw', 'o AND q -> r', 'dh AND dj -> dk', 'ap LSHIFT 1 -> bj', 'bk LSHIFT 1 -> ce', 'NOT ii -> ij', 'gh OR gi -> gj', 'kk RSHIFT 1 -> ld', 'lc LSHIFT 1 -> lw', 'lb OR la -> lc', '1 AND am -> an', 'gn AND gp -> gq', 'lf RSHIFT 3 -> lh', 'e OR f -> g', 'lg AND lm -> lo', 'ci RSHIFT 1 -> db', 'cf LSHIFT 1 -> cz', 'bn RSHIFT 1 -> cg', 'et AND fe -> fg', 'is OR it -> iu', 'kw AND ky -> kz', 'ck AND cl -> cn', 'bj OR bi -> bk', 'gj RSHIFT 1 -> hc', 'iu AND jf -> jh', 'NOT bs -> bt', 'kk OR kv -> kw', 'ks AND ku -> kv', 'hz OR ik -> il', 'b RSHIFT 1 -> v', 'iu RSHIFT 1 -> jn', 'fo RSHIFT 5 -> fr', 'be AND bg -> bh', 'ga AND gc -> gd', 'hf OR hl -> hm', 'ld OR le -> lf', 'as RSHIFT 5 -> av', 'fm OR fn -> fo', 'hm AND ho -> hp', 'lg OR lm -> ln', 'NOT kx -> ky', 'kk RSHIFT 3 -> km', 'ek AND em -> en', 'NOT ft -> fu', 'NOT jh -> ji', 'jn OR jo -> jp', 'gj AND gu -> gw', 'd AND j -> l', 'et RSHIFT 1 -> fm', 'jq OR jw -> jx', 'ep OR eo -> eq', 'lv LSHIFT 15 -> lz', 'NOT ey -> ez', 'jp RSHIFT 2 -> jq', 'eg AND ei -> ej', 'NOT dm -> dn', 'jp AND ka -> kc', 'as AND bd -> bf', 'fk OR fj -> fl', 'dw OR dx -> dy', 'lj AND ll -> lm', 'ec AND ee -> ef', 'fq AND fr -> ft', 'NOT kp -> kq', 'ki OR kj -> kk', 'cz OR cy -> da', 'as RSHIFT 3 -> au', 'an LSHIFT 15 -> ar', 'fj LSHIFT 15 -> fn', '1 AND fi -> fj', 'he RSHIFT 1 -> hx', 'lf RSHIFT 2 -> lg', 'kf LSHIFT 15 -> kj', 'dz AND ef -> eh', 'ib OR ic -> id', 'lf RSHIFT 5 -> li', 'bp OR bq -> br', 'NOT gs -> gt', 'fo RSHIFT 1 -> gh', 'bz AND cb -> cc', 'ea OR eb -> ec', 'lf AND lq -> ls', 'NOT l -> m', 'hz RSHIFT 3 -> ib', 'NOT di -> dj', 'NOT lk -> ll', 'jp RSHIFT 3 -> jr', 'jp RSHIFT 5 -> js', 'NOT bf -> bg', 's LSHIFT 15 -> w', 'eq LSHIFT 1 -> fk', 'jl OR jk -> jm', 'hz AND ik -> im', 'dz OR ef -> eg', '1 AND gy -> gz', 'la LSHIFT 15 -> le', 'br AND bt -> bu', 'NOT cn -> co', 'v OR w -> x', 'd OR j -> k', '1 AND gd -> ge', 'ia OR ig -> ih', 'NOT go -> gp', 'NOT ed -> ee', 'jq AND jw -> jy', 'et OR fe -> ff', 'aw AND ay -> az', 'ff AND fh -> fi', 'ir LSHIFT 1 -> jl', 'gg LSHIFT 1 -> ha', 'x RSHIFT 2 -> y', 'db OR dc -> dd', 'bl OR bm -> bn', 'ib AND ic -> ie', 'x RSHIFT 3 -> z', 'lh AND li -> lk', 'ce OR cd -> cf', 'NOT bb -> bc', 'hi AND hk -> hl', 'NOT gb -> gc', '1 AND r -> s', 'fw AND fy -> fz', 'fb AND fd -> fe', '1 AND en -> eo', 'z OR aa -> ab', 'bi LSHIFT 15 -> bm', 'hg OR hh -> hi', 'kh LSHIFT 1 -> lb', 'cg OR ch -> ci', '1 AND kz -> la', 'gf OR ge -> gg', 'gj RSHIFT 2 -> gk', 'dd RSHIFT 2 -> de', 'NOT ls -> lt', 'lh OR li -> lj', 'jr OR js -> jt', 'au AND av -> ax', '0 -> c', 'he AND hp -> hr', 'id AND if -> ig', 'et RSHIFT 5 -> ew', 'bp AND bq -> bs', 'e AND f -> h', 'ly OR lz -> ma', '1 AND lu -> lv', 'NOT jd -> je', 'ha OR gz -> hb', 'dy RSHIFT 1 -> er', 'iu RSHIFT 2 -> iv', 'NOT hr -> hs', 'as RSHIFT 1 -> bl', 'kk RSHIFT 2 -> kl', 'b AND n -> p', 'ln AND lp -> lq', 'cj AND cp -> cr', 'dl AND dn -> do', 'ci RSHIFT 2 -> cj', 'as OR bd -> be', 'ge LSHIFT 15 -> gi', 'hz RSHIFT 5 -> ic', 'dv LSHIFT 1 -> ep', 'kl OR kr -> ks', 'gj OR gu -> gv', 'he RSHIFT 5 -> hh', 'NOT fg -> fh', 'hg AND hh -> hj', 'b OR n -> o', 'jk LSHIFT 15 -> jo', 'gz LSHIFT 15 -> hd', 'cy LSHIFT 15 -> dc', 'kk RSHIFT 5 -> kn', 'ci RSHIFT 3 -> ck', 'at OR az -> ba', 'iu RSHIFT 3 -> iw', 'ko AND kq -> kr', 'NOT eh -> ei', 'aq OR ar -> as', 'iy AND ja -> jb', 'dd RSHIFT 3 -> df', 'bn RSHIFT 3 -> bp', '1 AND cc -> cd', 'at AND az -> bb', 'x OR ai -> aj', 'kk AND kv -> kx', 'ao OR an -> ap', 'dy RSHIFT 3 -> ea', 'x RSHIFT 1 -> aq', 'eu AND fa -> fc', 'kl AND kr -> kt', 'ia AND ig -> ii', 'df AND dg -> di', 'NOT fx -> fy', 'k AND m -> n', 'bn RSHIFT 5 -> bq', 'km AND kn -> kp', 'dt LSHIFT 15 -> dx', 'hz RSHIFT 2 -> ia', 'aj AND al -> am', 'cd LSHIFT 15 -> ch', 'hc OR hd -> he', 'he RSHIFT 3 -> hg', 'bn OR by -> bz', 'NOT kt -> ku', 'z AND aa -> ac', 'NOT ak -> al', 'cu AND cw -> cx', 'NOT ie -> if', 'dy RSHIFT 2 -> dz', 'ip LSHIFT 15 -> it', 'de OR dk -> dl', 'au OR av -> aw', 'jg AND ji -> jj', 'ci AND ct -> cv', 'dy RSHIFT 5 -> eb', 'hx OR hy -> hz', 'eu OR fa -> fb', 'gj RSHIFT 3 -> gl', 'fo AND fz -> gb', '1 AND jj -> jk', 'jp OR ka -> kb', 'de AND dk -> dm', 'ex AND ez -> fa', 'df OR dg -> dh', 'iv OR jb -> jc', 'x RSHIFT 5 -> aa', 'NOT hj -> hk', 'NOT im -> in', 'fl LSHIFT 1 -> gf', 'hu LSHIFT 15 -> hy', 'iq OR ip -> ir', 'iu RSHIFT 5 -> ix', 'NOT fc -> fd', 'NOT el -> em', 'ck OR cl -> cm', 'et RSHIFT 3 -> ev', 'hw LSHIFT 1 -> iq', 'ci RSHIFT 5 -> cl', 'iv AND jb -> jd', 'dd RSHIFT 5 -> dg', 'as RSHIFT 2 -> at', 'NOT jy -> jz', 'af AND ah -> ai', '1 AND ds -> dt', 'jx AND jz -> ka', 'da LSHIFT 1 -> du', 'fs AND fu -> fv', 'jp RSHIFT 1 -> ki', 'iw AND ix -> iz', 'iw OR ix -> iy', 'eo LSHIFT 15 -> es', 'ev AND ew -> ey', 'ba AND bc -> bd', 'fp AND fv -> fx', 'jc AND je -> jf', 'et RSHIFT 2 -> eu', 'kg OR kf -> kh', 'iu OR jf -> jg', 'er OR es -> et', 'fo RSHIFT 2 -> fp', 'NOT ca -> cb', 'bv AND bx -> by', 'u LSHIFT 1 -> ao', 'cm AND co -> cp', 'y OR ae -> af', 'bn AND by -> ca', '1 AND ke -> kf', 'jt AND jv -> jw', 'fq OR fr -> fs', 'dy AND ej -> el', 'NOT kc -> kd', 'ev OR ew -> ex', 'dd OR do -> dp', 'NOT cv -> cw', 'gr AND gt -> gu', 'dd RSHIFT 1 -> dw', 'NOT gw -> gx', 'NOT iz -> ja', '1 AND io -> ip', 'NOT ag -> ah', 'b RSHIFT 5 -> f', 'NOT cr -> cs', 'kb AND kd -> ke', 'jr AND js -> ju', 'cq AND cs -> ct', 'il AND in -> io', 'NOT ju -> jv', 'du OR dt -> dv', 'dd AND do -> dq', 'b RSHIFT 2 -> d', 'jm LSHIFT 1 -> kg', 'NOT dq -> dr', 'bo OR bu -> bv', 'gk OR gq -> gr', 'he OR hp -> hq', 'NOT h -> i', 'hf AND hl -> hn', 'gv AND gx -> gy', 'x AND ai -> ak', 'bo AND bu -> bw', 'hq AND hs -> ht', 'hz RSHIFT 1 -> is', 'gj RSHIFT 5 -> gm', 'g AND i -> j', 'gk AND gq -> gs', 'dp AND dr -> ds', 'b RSHIFT 3 -> e', 'gl AND gm -> go', 'gl OR gm -> gn', 'y AND ae -> ag', 'hv OR hu -> hw', '1674 -> b', 'ab AND ad -> ae', 'NOT ac -> ad', '1 AND ht -> hu', 'NOT hn -> ho' ],
testCommands = [ '123 -> x', '456 -> y', 'x AND y -> d', 'x OR y -> e', 'x LSHIFT 2 -> f', 'y RSHIFT 2 -> g', 'NOT x -> h', 'NOT y -> i' ];

function parseCommand( command ) {
	var parts,
	// x AND y -> z
	parseAssign = /^([a-z0-9]+) (RSHIFT|LSHIFT|AND|OR) ([a-z0-9]+) -> ([a-z0-9]+)$/,
	// 123 -> x
	assign = /^([a-z0-9]+) -> ([a-z0-9]+)$/,
	// NOT e -> f
	not = /^NOT ([a-z0-9]+) -> ([a-z0-9]+)$/;
	if ( parts = parseAssign.exec( command ) ) {
		return {
			action: parts[2],
			operator1: parts[1],
			operator2: parts[3],
			target: parts[4]
		};
	} else if ( parts = assign.exec( command ) ) {
		return {
			action: 'ASSIGN',
			operator: parts[1],
			target: parts[2]
		};
	} else if ( parts = not.exec( command ) ) {
		parts = not.exec( command );
		return {
			action: 'NOT',
			operator: parts[1],
			target: parts[2]
		};
	}

	return false;
}

function isWire( wire ) {
	return ( null !== wire.match( /[a-z]+/ ) );
}
function hasSignal() {}

function assign( operator, target ) {
	// Make sure that if it's a wire it's set before assigning.
	if ( isWire( operator ) && undefined === wires[ operator ] ) {
		return false;
	}

	if ( isWire( operator ) ) {
		wires[ target ] = wires[ operator ];
	} else {
		wires[ target ] = parseInt( operator, 10 );
	}
	return true;
}

function rshift( operator1, operator2, target ) {
	var op1, op2;
	if ( isWire( operator1 ) && undefined === wires[ operator1 ] ) {
		return false;
	} else if ( isWire( operator1 ) ) {
		op1 = wires[ operator1 ];
	} else {
		op1 = parseInt( operator1, 10 );
	}

	if ( isWire( operator2 ) && undefined === wires[ operator2 ] ) {
		return false;
	} else if ( isWire( operator2 ) ) {
		op2 = wires[ operator2 ];
	} else {
		op2 = parseInt( operator2, 10 );
	}

	wires[ target ] = op1 >> op2;
	return true;
}

function lshift( operator1, operator2, target ) {
	var op1, op2;
	if ( isWire( operator1 ) && undefined === wires[ operator1 ] ) {
		return false;
	} else if ( isWire( operator1 ) ) {
		op1 = wires[ operator1 ];
	} else {
		op1 = parseInt( operator1, 10 );
	}

	if ( isWire( operator2 ) && undefined === wires[ operator2 ] ) {
		return false;
	} else if ( isWire( operator2 ) ) {
		op2 = wires[ operator2 ];
	} else {
		op2 = parseInt( operator2, 10 );
	}

	wires[ target ] = op1 << op2;
	return true;
}

function and( operator1, operator2, target ) {
	var op1, op2;
	if ( isWire( operator1 ) && undefined === wires[ operator1 ] ) {
		return false;
	} else if ( isWire( operator1 ) ) {
		op1 = wires[ operator1 ];
	} else {
		op1 = parseInt( operator1, 10 );
	}

	if ( isWire( operator2 ) && undefined === wires[ operator2 ] ) {
		return false;
	} else if ( isWire( operator2 ) ) {
		op2 = wires[ operator2 ];
	} else {
		op2 = parseInt( operator2, 10 );
	}

	wires[ target ] = op1 & op2;
	return true;
}

function or( operator1, operator2, target ) {
	var op1, op2;
	if ( isWire( operator1 ) && undefined === wires[ operator1 ] ) {
		return false;
	} else if ( isWire( operator1 ) ) {
		op1 = wires[ operator1 ];
	} else {
		op1 = parseInt( operator1, 10 );
	}

	if ( isWire( operator2 ) && undefined === wires[ operator2 ] ) {
		return false;
	} else if ( isWire( operator2 ) ) {
		op2 = wires[ operator2 ];
	} else {
		op2 = parseInt( operator2, 10 );
	}

	wires[ target ] = op1 | op2;
	return true;
}

function not( operator, target ) {
	var op1;
	if ( isWire( operator ) && undefined === wires[ operator ] ) {
		return false;
	} else if ( isWire( operator ) ) {
		op1 = wires[ operator ];
	} else {
		op1 = parseInt( operator, 10 );
	}

	wires[ target ] = op1 ^ 65535; // Takes care of NOT for 16-bit unsigned.
	return true;
}

while ( commands.length ) {
	var success,
	cmd = commands.shift(),
	command = parseCommand( cmd );
	switch ( command.action ) {
		case 'ASSIGN':
			success = assign( command.operator, command.target );
			break;

		case 'RSHIFT':
			success = rshift( command.operator1, command.operator2, command.target );
			break;

		case 'LSHIFT':
			success = lshift( command.operator1, command.operator2, command.target );
			break;

		case 'AND':
			success = and( command.operator1, command.operator2, command.target );
			break;

		case 'OR':
			success = or( command.operator1, command.operator2, command.target );
			break;

		case 'NOT':
			success = not( command.operator, command.target );
			break;
	}

	if ( ! success ) {
		commands.push( cmd );
	}
}

console.log( wires );