Map a continuous value within a certain domain to a discrete value in JavaScript. Great for maps where you need to find a certain color for a given value.

quantize.js

const quantizeScale = (arr, min, max) => {

  const steps = colors.length;
  const increment = (max - min) / steps;
  const bins = Array.apply(undefined, Array(steps)).map((bin, index) => (index * increment) + min);

  return value => {
    for (let i = 0; i < bins.length; i++) {
      if (value <= bins[i]) {
        return colors[i];
      }
    }
    return colors[colors.length - 1];
  };
};

// Discrete color scale from light blue to dark blue
const colors = ['#f1eef6', '#bdc9e1', '#74a9cf', '#2b8cbe', '#045a8d'];
const minimum = 5;
const maximum = 20;

// Construct new scale
const quantize = quantizeScale(colors, minimum, maximum);

console.log(quantize(0)); // => #f1eef6
console.log(quantize(5)); // => #f1eef6
console.log(quantize(10)); // => #74a9cf
console.log(quantize(15)); // => #045a8d
console.log(quantize(20)); // => #045a8d
console.log(quantize(100)); // => #045a8d