codetest.js

for (var i = 0; i < 100; i++) {
  var denominator = intensiveFunction();

  var numerator = asynchronousFunction(i);

  var fractions = [];

  if (i < 10) fractions.push(numerator / denominator);
  else if (i < 30) fractions.push((numerator * 2) / denominator);
  else if (i < 90) fractions.push((numerator / 2) / denominator);
  else if (i < 100) fractions.push((numerator) / (2 * denominator));
}

for (var i = 0; i < 100; i++) {
  console.log(fractions[i]);
}

/*
Optimisations:
Calculate denominator once outside of first for loop
Remove nested ifs and replace with 4 for loops

Bugs:
Fractions is out of scope in second for loop
numerator could be undefined, push should be in a callback to asynchronousFunction
*/

• Generally, I think it would be better to give a more concrete example rather than 'intensiveFunction' etc
• Not having any evidence of callback support in 'asynchronousFunction' is too misleading, and anyway using an asynchronous function inside a for loop is a non-starter.
• Even the single-line if statements should use braces (according to Toby and Douglas Crockford :) )
• Use ES6 syntax? Otherwise I don't think 'fractions' is out of scope in second for loop (see variable hoisting)