J - Z 系列

获取数组元素下标（findIndex的实现）

const linearSearch = (arr, item) => {
  for (const i in arr) {
    if (arr[i] === item) return +i;
  }
  return -1;
};

linearSearch([2, 9, 9], 9); // 1
linearSearch([2, 9, 9], 7); // -1
// 只返回数组中的第一个

绑定事件一次（前面也有类似函数）

const listenOnce = (el, evt, fn) =>
  el.addEventListener(evt, fn, { once: true });
  // 可以传入 option（对象） 也可以传入 useCapture（布尔）

listenOnce(
  document.getElementById('my-id'),
  'click',
  () => console.log('Hello world')
); // 'Hello world' will only be logged on the first click

给定键值对，转换为对象

const objectFromPairs = arr =>
  // 结构赋值，直接获取了 key和value
  arr.reduce((a, [key, val]) => ((a[key] = val), a), {});

objectFromPairs([['a', 1], ['b', 2]]); // {a: 1, b: 2}

检测用户是否是暗黑模式

const prefersDarkColorScheme = () =>
  window &&
  window.matchMedia &&
  window.matchMedia('(prefers-color-scheme: dark)').matches;

prefersDarkColorScheme(); // true

将字节单位大小转换为 KB,MB等

const prettyBytes = (num, precision = 3, addSpace = true) => {
  const UNITS = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
  if (Math.abs(num) < 1) return num + (addSpace ? ' ' : '') + UNITS[0];
  const exponent = Math.min(
    Math.floor(Math.log10(num < 0 ? -num : num) / 3),
    UNITS.length - 1
  );
  const n = Number(
    ((num < 0 ? -num : num) / 1000 ** exponent).toPrecision(precision)
  );
  return (num < 0 ? '-' : '') + n + (addSpace ? ' ' : '') + UNITS[exponent];
};

prettyBytes(1000); // '1 KB'
prettyBytes(-27145424323.5821, 5); // '-27.145 GB'
prettyBytes(123456789, 3, false); // '123MB'

返回提供日期的季度

const quarterOfYear = (date = new Date()) => [
  Math.ceil((date.getMonth() + 1) / 3),
  date.getFullYear()
];

quarterOfYear(new Date('07/10/2018')); // [ 3, 2018 ]
quarterOfYear(); // [ 4, 2020 ]

将url问号后参数转换为键值对

const queryStringToObject = url =>
  [...new URLSearchParams(url.split('?')[1])].reduce(
    (a, [k, v]) => ((a[k] = v), a),
    {}
  );

queryStringToObject('https://google.com?page=1&count=10');
// {page: '1', count: '10'}

数组快速排序

const quickSort = arr => {
  const a = [...arr];
  if (a.length < 2) return a;
  // 向下取整
  const pivotIndex = Math.floor(arr.length / 2);
  const pivot = a[pivotIndex];
  const [lo, hi] = a.reduce(
    (acc, val, i) => {
      if (val < pivot || (val === pivot && i != pivotIndex)) {
        acc[0].push(val);
      } else if (val > pivot) {
        acc[1].push(val);
      }
      return acc;
    },
    [[], []]
  );
  return [...quickSort(lo), pivot, ...quickSort(hi)];
};

quickSort([1, 6, 1, 5, 3, 2, 1, 4]); // [1, 1, 1, 2, 3, 4, 5, 6]

生成指定长度随机字符串

const randomAlphaNumeric = length => {
  let s = '';
  Array.from({ length }).some(() => {
    s += Math.random().toString(36).slice(2);
    return s.length >= length;
  });
  return s.slice(0, length);
};

randomAlphaNumeric(5); // '0afad'

随机16进制颜色

const randomHexColorCode = () => {
  let n = (Math.random() * 0xfffff * 1000000).toString(16);
  return '#' + n.slice(0, 6);
};

randomHexColorCode(); // '#e34155'

随机范围数组

const randomIntArrayInRange = (min, max, n = 1) =>
  Array.from(
    { length: n },
    () => Math.floor(Math.random() * (max - min + 1)) + min
  );

randomIntArrayInRange(12, 35, 10); // [ 34, 14, 27, 17, 30, 27, 20, 26, 21, 14 ]

随机数

const randomIntegerInRange = (min, max) =>
  Math.floor(Math.random() * (max - min + 1)) + min;

randomIntegerInRange(0, 5); // 2

创建一个生成器，该生成器使用给定的步长生成给定范围内的所有值

const rangeGenerator = function* (start, end, step = 1) {
  let i = start;
  while (i < end) {
    yield i;
    i += step;
  }
};

// for of 用于遍历集合 （迭代器对象）
for (let i of rangeGenerator(6, 10)) console.log(i);
// Logs 6, 7, 8, 9

requestAnimationFrame 动画帧的形式，调用回调，同时提供取消方法

const recordAnimationFrames = (callback, autoStart = true) => {
  let running = false,
    raf;
  const stop = () => {
    if (!running) return;
    running = false;
    cancelAnimationFrame(raf);
  };
  const start = () => {
    if (running) return;
    running = true;
    run();
  };
  const run = () => {
    raf = requestAnimationFrame(() => {
      callback();
      if (running) run();
    });
  };
  if (autoStart) start();
  return { start, stop };
};

const cb = () => console.log('Animation frame fired');
const recorder = recordAnimationFrames(cb);
// logs 'Animation frame fired' on each animation frame
recorder.stop(); // stops logging
recorder.start(); // starts again
const recorder2 = recordAnimationFrames(cb, false);
// `start` needs to be explicitly called to begin recording frames

根据提供函数，过滤数组对象，并且保留需要的键值对

const reducedFilter = (data, keys, fn) =>
  data.filter(fn).map(el =>
    keys.reduce((acc, key) => {
      acc[key] = el[key];
      return acc;
    }, {})
  );

const data = [
  {
    id: 1,
    name: 'john',
    age: 24
  },
  {
    id: 2,
    name: 'mike',
    age: 50
  }
];
reducedFilter(data, ['id', 'name'], item => item.age > 24);
// [{ id: 2, name: 'mike'}]
// 有点意思，将保留的键数组,利用reduce再重组下

回到顶部，带动画

const scrollToTop = () => {
  const c = document.documentElement.scrollTop || document.body.scrollTop;
  if (c > 0) {
    window.requestAnimationFrame(scrollToTop);
    window.scrollTo(0, c - c / 8);
  }
};

// requestAnimationFrame api的运用，同时可以扩展（滚动到指定元素或者其他方向）
//  c - c / 8 距离的计算可以学习下（类似缓动动画？）

数组打乱顺序

const shuffle = ([...arr]) => {
  let m = arr.length;
  while (m) {
    const i = Math.floor(Math.random() * m--);
    // 变量交换位置
    [arr[m], arr[i]] = [arr[i], arr[m]];
  }
  return arr;
};

const foo = [1, 2, 3];
shuffle(foo); // [2, 3, 1], foo = [1, 2, 3]

去掉html标签

const stripHTMLTags = str => str.replace(/<[^>]*>/g, '');

stripHTMLTags('<p><em>lorem</em> <strong>ipsum</strong></p>'); // 'lorem ipsum'

reduce的求和

const sum = (...arr) => [...arr].reduce((acc, val) => acc + val, 0);

sum(1, 2, 3, 4); // 10
sum(...[1, 2, 3, 4]); // 10