node 中的 nextTick

node 中的 nextTick 是 node 自带全局的变量 process 的一个方法，process.nextTick 是一个微任务，在 node 的所有微任务中最先执行，是优先级最高的微任务。浏览器中是没有这一个方法的。

vue 中的 nextTick

vue 中的 nextTick 和 node 中的完全不同的东西，是 vue 源码中有自己的实现方法，而且 vue2 和 vue3 中的实现方法还不同。作用是在下次 DOM 更新循环结束之后执行延迟回调，

在下次 DOM 更新循环结束之后执行延迟回调

vue2 中的 nextTick 实现方法

在 vue2 源码中有一个专门的文件用来实现 nextTick 方法，可以自己去看一下，他依次判断并使用了 promise、mutationObserver、setImmediate、setTimeout 来调用回调函数

源代码如下

/* globals MutationObserver */import { noop } from 'shared/util'
import { handleError } from './error'
import { isIE, isIOS, isNative } from './env'export let isUsingMicroTask = falseconst callbacks: Array<Function> = []
let pending = falsefunction flushCallbacks() {pending = falseconst copies = callbacks.slice(0)callbacks.length = 0for (let i = 0; i < copies.length; i++) {copies[i]()}
}// Here we have async deferring wrappers using microtasks.
// In 2.5 we used (macro) tasks (in combination with microtasks).
// However, it has subtle problems when state is changed right before repaint
// (e.g. #6813, out-in transitions).
// Also, using (macro) tasks in event handler would cause some weird behaviors
// that cannot be circumvented (e.g. #7109, #7153, #7546, #7834, #8109).
// So we now use microtasks everywhere, again.
// A major drawback of this tradeoff is that there are some scenarios
// where microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690, which have workarounds)
// or even between bubbling of the same event (#6566).
let timerFunc// The nextTick behavior leverages the microtask queue, which can be accessed
// via either native Promise.then or MutationObserver.
// MutationObserver has wider support, however it is seriously bugged in
// UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It
// completely stops working after triggering a few times... so, if native
// Promise is available, we will use it:
/* istanbul ignore next, $flow-disable-line */
if (typeof Promise !== 'undefined' && isNative(Promise)) {const p = Promise.resolve()timerFunc = () => {p.then(flushCallbacks)// In problematic UIWebViews, Promise.then doesn't completely break, but// it can get stuck in a weird state where callbacks are pushed into the// microtask queue but the queue isn't being flushed, until the browser// needs to do some other work, e.g. handle a timer. Therefore we can// "force" the microtask queue to be flushed by adding an empty timer.if (isIOS) setTimeout(noop)}isUsingMicroTask = true
} else if (!isIE &&typeof MutationObserver !== 'undefined' &&(isNative(MutationObserver) ||// PhantomJS and iOS 7.xMutationObserver.toString() === '[object MutationObserverConstructor]')
) {// Use MutationObserver where native Promise is not available,// e.g. PhantomJS, iOS7, Android 4.4// (#6466 MutationObserver is unreliable in IE11)let counter = 1const observer = new MutationObserver(flushCallbacks)const textNode = document.createTextNode(String(counter))observer.observe(textNode, {characterData: true})timerFunc = () => {counter = (counter + 1) % 2textNode.data = String(counter)}isUsingMicroTask = true
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {// Fallback to setImmediate.// Technically it leverages the (macro) task queue,// but it is still a better choice than setTimeout.timerFunc = () => {setImmediate(flushCallbacks)}
} else {// Fallback to setTimeout.timerFunc = () => {setTimeout(flushCallbacks, 0)}
}export function nextTick(): Promise<void>
export function nextTick<T>(this: T, cb: (this: T, ...args: any[]) => any): void
export function nextTick<T>(cb: (this: T, ...args: any[]) => any, ctx: T): void
/*** @internal*/
export function nextTick(cb?: (...args: any[]) => any, ctx?: object) {let _resolvecallbacks.push(() => {if (cb) {try {cb.call(ctx)} catch (e: any) {handleError(e, ctx, 'nextTick')}} else if (_resolve) {_resolve(ctx)}})if (!pending) {pending = truetimerFunc()}// $flow-disable-lineif (!cb && typeof Promise !== 'undefined') {return new Promise(resolve => {_resolve = resolve})}
}

vue3 中 nextTick 实现方法

vue3 中使用 promise 来实现，nextTick 方法返回一个 Promise 对象，因此可以使用 Promise 的链式调用或 async/await 语法来处理 nextTick 回调。在源码的scheduler.ts 文件中定义

完整代码如下

import { ErrorCodes, callWithErrorHandling, handleError } from './errorHandling'
import { type Awaited, NOOP, isArray } from '@vue/shared'
import { type ComponentInternalInstance, getComponentName } from './component'export interface SchedulerJob extends Function {id?: numberpre?: booleanactive?: booleancomputed?: boolean/*** Indicates whether the effect is allowed to recursively trigger itself* when managed by the scheduler.** By default, a job cannot trigger itself because some built-in method calls,* e.g. Array.prototype.push actually performs reads as well (#1740) which* can lead to confusing infinite loops.* The allowed cases are component update functions and watch callbacks.* Component update functions may update child component props, which in turn* trigger flush: "pre" watch callbacks that mutates state that the parent* relies on (#1801). Watch callbacks doesn't track its dependencies so if it* triggers itself again, it's likely intentional and it is the user's* responsibility to perform recursive state mutation that eventually* stabilizes (#1727).*/allowRecurse?: boolean/*** Attached by renderer.ts when setting up a component's render effect* Used to obtain component information when reporting max recursive updates.* dev only.*/ownerInstance?: ComponentInternalInstance
}export type SchedulerJobs = SchedulerJob | SchedulerJob[]let isFlushing = false
let isFlushPending = falseconst queue: SchedulerJob[] = []
let flushIndex = 0const pendingPostFlushCbs: SchedulerJob[] = []
let activePostFlushCbs: SchedulerJob[] | null = null
let postFlushIndex = 0const resolvedPromise = /*#__PURE__*/ Promise.resolve() as Promise<any>
let currentFlushPromise: Promise<void> | null = nullconst RECURSION_LIMIT = 100
type CountMap = Map<SchedulerJob, number>export function nextTick<T = void, R = void>(this: T,fn?: (this: T) => R,
): Promise<Awaited<R>> {const p = currentFlushPromise || resolvedPromisereturn fn ? p.then(this ? fn.bind(this) : fn) : p
}// #2768
// Use binary-search to find a suitable position in the queue,
// so that the queue maintains the increasing order of job's id,
// which can prevent the job from being skipped and also can avoid repeated patching.
function findInsertionIndex(id: number) {// the start index should be `flushIndex + 1`let start = flushIndex + 1let end = queue.lengthwhile (start < end) {const middle = (start + end) >>> 1const middleJob = queue[middle]const middleJobId = getId(middleJob)if (middleJobId < id || (middleJobId === id && middleJob.pre)) {start = middle + 1} else {end = middle}}return start
}export function queueJob(job: SchedulerJob) {// the dedupe search uses the startIndex argument of Array.includes()// by default the search index includes the current job that is being run// so it cannot recursively trigger itself again.// if the job is a watch() callback, the search will start with a +1 index to// allow it recursively trigger itself - it is the user's responsibility to// ensure it doesn't end up in an infinite loop.if (!queue.length ||!queue.includes(job,isFlushing && job.allowRecurse ? flushIndex + 1 : flushIndex,)) {if (job.id == null) {queue.push(job)} else {queue.splice(findInsertionIndex(job.id), 0, job)}queueFlush()}
}function queueFlush() {if (!isFlushing && !isFlushPending) {isFlushPending = truecurrentFlushPromise = resolvedPromise.then(flushJobs)}
}export function invalidateJob(job: SchedulerJob) {const i = queue.indexOf(job)if (i > flushIndex) {queue.splice(i, 1)}
}export function queuePostFlushCb(cb: SchedulerJobs) {if (!isArray(cb)) {if (!activePostFlushCbs ||!activePostFlushCbs.includes(cb,cb.allowRecurse ? postFlushIndex + 1 : postFlushIndex,)) {pendingPostFlushCbs.push(cb)}} else {// if cb is an array, it is a component lifecycle hook which can only be// triggered by a job, which is already deduped in the main queue, so// we can skip duplicate check here to improve perfpendingPostFlushCbs.push(...cb)}queueFlush()
}export function flushPreFlushCbs(instance?: ComponentInternalInstance,seen?: CountMap,// if currently flushing, skip the current job itselfi = isFlushing ? flushIndex + 1 : 0,
) {if (__DEV__) {seen = seen || new Map()}for (; i < queue.length; i++) {const cb = queue[i]if (cb && cb.pre) {if (instance && cb.id !== instance.uid) {continue}if (__DEV__ && checkRecursiveUpdates(seen!, cb)) {continue}queue.splice(i, 1)i--cb()}}
}export function flushPostFlushCbs(seen?: CountMap) {if (pendingPostFlushCbs.length) {const deduped = [...new Set(pendingPostFlushCbs)].sort((a, b) => getId(a) - getId(b),)pendingPostFlushCbs.length = 0// #1947 already has active queue, nested flushPostFlushCbs callif (activePostFlushCbs) {activePostFlushCbs.push(...deduped)return}activePostFlushCbs = dedupedif (__DEV__) {seen = seen || new Map()}for (postFlushIndex = 0;postFlushIndex < activePostFlushCbs.length;postFlushIndex++) {if (__DEV__ &&checkRecursiveUpdates(seen!, activePostFlushCbs[postFlushIndex])) {continue}activePostFlushCbs[postFlushIndex]()}activePostFlushCbs = nullpostFlushIndex = 0}
}const getId = (job: SchedulerJob): number =>job.id == null ? Infinity : job.idconst comparator = (a: SchedulerJob, b: SchedulerJob): number => {const diff = getId(a) - getId(b)if (diff === 0) {if (a.pre && !b.pre) return -1if (b.pre && !a.pre) return 1}return diff
}function flushJobs(seen?: CountMap) {isFlushPending = falseisFlushing = trueif (__DEV__) {seen = seen || new Map()}// Sort queue before flush.// This ensures that:// 1. Components are updated from parent to child. (because parent is always//    created before the child so its render effect will have smaller//    priority number)// 2. If a component is unmounted during a parent component's update,//    its update can be skipped.queue.sort(comparator)// conditional usage of checkRecursiveUpdate must be determined out of// try ... catch block since Rollup by default de-optimizes treeshaking// inside try-catch. This can leave all warning code unshaked. Although// they would get eventually shaken by a minifier like terser, some minifiers// would fail to do that (e.g. https://github.com/evanw/esbuild/issues/1610)const check = __DEV__? (job: SchedulerJob) => checkRecursiveUpdates(seen!, job): NOOPtry {for (flushIndex = 0; flushIndex < queue.length; flushIndex++) {const job = queue[flushIndex]if (job && job.active !== false) {if (__DEV__ && check(job)) {continue}callWithErrorHandling(job, null, ErrorCodes.SCHEDULER)}}} finally {flushIndex = 0queue.length = 0flushPostFlushCbs(seen)isFlushing = falsecurrentFlushPromise = null// some postFlushCb queued jobs!// keep flushing until it drains.if (queue.length || pendingPostFlushCbs.length) {flushJobs(seen)}}
}function checkRecursiveUpdates(seen: CountMap, fn: SchedulerJob) {if (!seen.has(fn)) {seen.set(fn, 1)} else {const count = seen.get(fn)!if (count > RECURSION_LIMIT) {const instance = fn.ownerInstanceconst componentName = instance && getComponentName(instance.type)handleError(`Maximum recursive updates exceeded${componentName ? ` in component <${componentName}>` : ``}. ` +`This means you have a reactive effect that is mutating its own ` +`dependencies and thus recursively triggering itself. Possible sources ` +`include component template, render function, updated hook or ` +`watcher source function.`,null,ErrorCodes.APP_ERROR_HANDLER,)return true} else {seen.set(fn, count + 1)}}
}