Utilities

utils.js

// @flow

/// MATHSY STUFF ///
// constrain a value between min and max
export const constrain = (min: number, max: number, n: number): number =>
  Math.max(min, Math.min(max, n));

// covert degrees into radians
export const degreesToRadians = (degrees: number): number =>
  degrees => degrees / (180 / Math.PI);

// convert radians to degrees
export const radiansToDegrees = (radians: number): number =>
  radians * (180 / Math.PI);

// linear interpolate between a and b
export const lerp = (a: number, b: number, n): number =>
  a + (b - a) * n;

// adjust the range of a number.
// e.g. mapRange(0, 1, 0, 100, 0.5) === 50
//      mapRange(5, 10, 0, 10, 6) === 2
export const mapRange = (min1: number, max1: number, min2: number, max2: number, n: number) => {
  const progress = (n - min1) / (max1 - min1);
  return lerp(min2, max2, progress);
};


// calculate the mean of an array of numbers
export const mean (arr: number[]): number => {
  const sum = arr.reduce((a, b) => a + b, 0);
  return sum / arr.length;
};

/// NON MATHSY STUFF ///

// remove all the null-ish values from an array
export const compact = <T>(arr: Array<?T>): Array<T> => {
  // $FlowFixMe flow can't tell that flow does a type refinement
  return arr.filter(item => item != null);
}

// returns a promise that will resolve after `msTimeout` milliseconds
export const delay = (msTimeout: number): Promise<void> =>
  new Promise(resolve => setTimeout(resolve, msTimeout));

// a function wrapped with lazy will only ever get called once. every
// subsequent call returns a cached value
export const lazy = <T>(fn: () => T): () => T => {
  let cache;
  let called = false;

  return () => {
    if (!called) {
      called = true;
      cache = fn();
    }

    return cache;
  };
};

const getNowFn = (): () => number => {
  let now = () => Date.now();

  if (
    typeof window !== 'undefined' &&
    window.performance &&
    window.performance.now
  ) {
    now = () => window.performance.now();
  }

  return now;
}

// gets ms time. tries to use performance.now(), but falls back to Date.now()
export const now = getNowFn();

// debounce a function to the next requestAnimationFrame call
export const rafDebounce = (fn: () => void): (() => void) => {
  let called = false;
  let lastArgs = [];

  return (...args) => {
    lastArgs = args;

    if (called) return;
    called = true;
    window.requestAnimationFrame(() => {
      called = false;
      lastArgs = [];
      fn(...lastArgs);
    });
  };
};

// call fn n times, and return an array of all the return values
export const times = <T>(n: number, fn: (number) => T): T[] => {
  const results = [];

  for (let i = 0; i < n; i++) {
    results.push(fn(i));
  }

  return results;
};