Simplifying complex conditional expressions

conditionals.js

/**
 * The problem
 */

var A = true; 
var B = true;
var C = false;
var D = false;

if ( A && B || C && ! D ) {
  // If you ever need to update this conditional, you're screwed
}

/**
 * The solution
 * 
 * First, group things together in new variables, improving your previous variable names.
 * If you're going to use this elsewhere, an even better idea is to break it out into a 
 * separate function. 
 */
 
var timerWorks = A && B;
var batterIsLow = !C || D;

var D = A && B;
if ( timerWorks && !batteryIsLow ) {
  // see how much easier it is to know when this condition will pass?
}