模拟实现

2023年06月11日

柏拉文

越努力，越幸运

一、实现

---

let pending = false;
const callbacks = [];

function isNative(Ctor) {
  return typeof Ctor === 'function' && /native code/.test(Ctor.toString());
}

function flushCallbacks() {
  pending = false;
  const copies = callbacks.slice();
  callbacks.length = 0;
  for (let i = 0; i < copies.length; i++) {
    copies[i]();
  }
}

let timerFunc;

if (typeof Promise !== 'undefined' && isNative(Promise)) {
  const p = Promise.resolve();
  timerFunc = () => {
    p.then(flushCallbacks);
  };
} else if (
  typeof MutationObserver !== 'undefined' &&
  isNative(MutationObserver)
) {
  let counter = 1;
  const observer = new MutationObserver(flushCallbacks);
  const textNode = document.createTextNode(String(counter));
  observer.observe(textNode, {
    characterData: true
  });

  timerFunc = () => {
    counter = (counter + 1) % 2;
    textNode.data = String(counter);
  };
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
  timerFunc = () => {
    setImmediate(flushCallbacks);
  };
} else {
  timerFunc = () => {
    setTimeout(flushCallbacks);
  };
}

function nextTick(cb, ctx) {
  let _resolve;
  callbacks.push(() => {
    if (cb) {
      try {
        cb.call(ctx);
      } catch (error) {
        console.log('nextTick 函数发生错误');
      }
    } else if (_resolve) {
      _resolve(ctx);
    }
  });

  if (!pending) {
    pending = true;
    timerFunc();
  }

  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve;
    });
  }
}

二、测试

---