其他js相关

更准确的 setInterval

setTimeout +系统时间补偿

// 10秒倒计时
let countdown = 10;
function timer() {
  let speed = 1000,
    counter = 1,
    start = Date.now();
  function timer_() {
    if (countdown === 0) {
      return;
    }
    const real = counter * speed,
      ideal = Date.now() - start;
    counter++;
    const diff = ideal - real;
    countdown--;
    setTimeout(() => {
      timer_();
    }, speed - diff);
  }
  setTimeout(() => {
    timer_();
  }, speed);
}

safeGet

const safeGet = (o, path) => {
  try {
    return path.split(".").reduce((o, k) => o[k], o);
  } catch (error) {
    return undefined;
  }
};

模拟实现 Object.freeze

function myFreeze(obj) {
  if (obj instanceof Object) {
    Object.seal(obj);
    let p;
    for (p in obj) {
      if (obj.hasOwnProperty(p)) {
        Object.defineProperty(obj, p, { writable: false });
        myFreeze(obj[p]);
      }
    }
  }
}

深冻结。freeze 是浅冻结

function myFreeze(obj) {
  Object.freeze(obj);
  for (const key in obj) {
    if (Object.prototype.hasOwnProperty.call(obj, key)) {
      const item = obj[key];
      if (typeof item === "object" && item !== null) {
        myFreeze(item);
      }
    }
  }
  return obj;
}
const person = {
  name: "Lance",
  grade: {
    Math: 100,
    IT: 90,
    Chinese: 100,
  },
  a: {
    b: {
      c: 1,
    },
  },
};

myFreeze(person);

person.a.b.c = 2333;
console.log(person);

模拟实现 let 和 const

let

function outputNum(count) {
  (function () {
    for (var i = 0; i < count; i++) {
      console.log(i);
    }
  })();
  console.log(i);
}
outputNum(5);

const

function _const(key, value) {
  window[key] = value;
  Object.defineProperty(window, key, {
    enumerable: false,
    configurable: false,
    get: function () {
      return value;
    },
    set: function (newValue) {
      if (newValue !== value) {
        throw TypeError("这是只读变量，不可修改");
      } else {
        return value;
      }
    },
  });
}