源码地址：vue-next/packages/shared/src/index.ts
学习链接：juejin.cn/post/699497…

1、源码

1、EMPTY_OBJ 可读的空对象

export const EMPTY_OBJ: { readonly [key: string]: any } = __DEV__
  ? Object.freeze({})
  : {}
//Object.freeze 是 冻结对象  冻结的对象最外层无法修改

2、EMPTY_ARR 空数组

export const EMPTY_ARR = __DEV__ ? Object.freeze([]) : []

3、NOOP 空函数与NO 永远返回 false 的函数

export const NOOP = () => {}
export const NO = () => false
场景：1、方便判断，2、便于压缩

// 1. 比如：
const instance = {
    render: NOOP
};

// 条件
const dev = true;
if(dev){
    instance.render = function(){
        console.log('render');
    }
}

// 可以用作判断。
if(instance.render === NOOP){
 console.log('i');
}
// 2. 再比如：
// 方便压缩代码
// 如果是 function(){} ，不方便压缩代码

4、isOn是否是on开头，且on首字母小写

const onRE = /^on[^a-z]/
export const isOn = (key: string) => onRE.test(key)

5、isModelListener 监听器 以onUpdate:开头

export const isModelListener = (key: string) => key.startsWith('onUpdate:')

6、extend 继承合并-浅拷贝

export const extend = Object.assign

const data = { name: '若川' };
const data2 = extend(data, { mp: '若川视野', name: '是若川啊' });
console.log(data); // { name: "是若川啊", mp: "若川视野" }
console.log(data2); // { name: "是若川啊", mp: "若川视野" }
console.log(data === data2); // true

7、删除数组某一项

export const remove = <T>(arr: T[], el: T) => {
  const i = arr.indexOf(el)
  if (i > -1) {
    arr.splice(i, 1)
  }
}

8、是否是自身的属性

const hasOwnProperty = Object.prototype.hasOwnProperty
export const hasOwn = (
  val: object,
  key: string | symbol
): key is keyof typeof val => hasOwnProperty.call(val, key)

//__proto__实例对象上的方法  原型链向上查找
hasOwn({__proto__: { a: 1 }}, 'a') // false
hasOwn({ a: undefined }, 'a') // true

9、类型判断

export const isArray = Array.isArray
export const isMap = (val: unknown): val is Map<any, any> =>
  toTypeString(val) === '[object Map]'
export const isSet = (val: unknown): val is Set<any> =>
  toTypeString(val) === '[object Set]'

export const isDate = (val: unknown): val is Date => val instanceof Date
export const isFunction = (val: unknown): val is Function =>
  typeof val === 'function'
export const isString = (val: unknown): val is string => typeof val === 'string'
export const isSymbol = (val: unknown): val is symbol => typeof val === 'symbol'
export const isObject = (val: unknown): val is Record<any, any> =>
  val !== null && typeof val === 'object'

export const isPromise = <T = any>(val: unknown): val is Promise<T> => {
  return isObject(val) && isFunction(val.then) && isFunction(val.catch)
}

10、类型

export const objectToString = Object.prototype.toString
export const toTypeString = (value: unknown): string =>
  objectToString.call(value)

export const toRawType = (value: unknown): string => {
  // extract "RawType" from strings like "[object RawType]"
  return toTypeString(value).slice(8, -1)
}

export const isPlainObject = (val: unknown): val is object =>
  toTypeString(val) === '[object Object]'

11、isIntegerKey是否是数字型字符串

export const isIntegerKey = (key: unknown) =>
  isString(key) &&
  key !== 'NaN' &&
  key[0] !== '-' &&
  '' + parseInt(key, 10) === key

12、保留属性与指令

export const isReservedProp = /*#__PURE__*/ makeMap(
  // the leading comma is intentional so empty string "" is also included
  ',key,ref,ref_for,ref_key,' +
    'onVnodeBeforeMount,onVnodeMounted,' +
    'onVnodeBeforeUpdate,onVnodeUpdated,' +
    'onVnodeBeforeUnmount,onVnodeUnmounted'
)

export const isBuiltInDirective = /*#__PURE__*/ makeMap(
  'bind,cloak,else-if,else,for,html,if,model,on,once,pre,show,slot,text,memo'
)

13、缓存

const cacheStringFunction = <T extends (str: string) => string>(fn: T): T => {
  const cache: Record<string, string> = Object.create(null)
  return ((str: string) => {
    const hit = cache[str]
    return hit || (cache[str] = fn(str))
  }) as any
}

const camelizeRE = /-(\w)/g
/**
 * @private
 */
export const camelize = cacheStringFunction((str: string): string => {
  return str.replace(camelizeRE, (_, c) => (c ? c.toUpperCase() : ''))
})

const hyphenateRE = /\B([A-Z])/g
/**
 * @private
 */
export const hyphenate = cacheStringFunction((str: string) =>
  str.replace(hyphenateRE, '-$1').toLowerCase()
)

/**
 * @private
 */
export const capitalize = cacheStringFunction(
  (str: string) => str.charAt(0).toUpperCase() + str.slice(1)
)

/**
 * @private
 */
export const toHandlerKey = cacheStringFunction((str: string) =>
  str ? `on${capitalize(str)}` : ``
)

14、hasChanged是否变化

// compare whether a value has changed, accounting for NaN.
export const hasChanged = (value: any, oldValue: any): boolean =>
  !Object.is(value, oldValue)

Object.is(value1, value2) (ES6) 与===区别：+0 === -0 NaN === NaN

Object.is('若川', '若川'); // true
Object.is({},{}); // false
Object.is(+0, -0); // false
+0 === -0; // true
Object.is(NaN, NaN); // true
NaN === NaN; // false
源码：
Object.defineProperty(Object, 'is', {
    value: function() {x, y} {
        if (x === y) {
           // 针对+0不等于-0的情况
           return x !== 0 || 1 / x === 1 / y;
        }
        // 针对 NaN的情况
        return x !== x && y !== y;
    },
    configurable: true,
    enumerable: false,
    writable: true
});

15、invokeArrayFns指向数组里面的函数

export const invokeArrayFns = (fns: Function[], arg?: any) => {
  for (let i = 0; i < fns.length; i++) {
    fns[i](arg)
  }
}

16、def定义对象属性

export const def = (obj: object, key: string | symbol, value: any) => {
  Object.defineProperty(obj, key, {
    configurable: true,
    enumerable: false,
    value
  })
}

17、toNumber转数字

export const toNumber = (val: any): any => {
  const n = parseFloat(val)
  return isNaN(n) ? val : n
}

18、getGlobalThis得到全局变量

let _globalThis: any
export const getGlobalThis = (): any => {
  return (
    _globalThis ||
    (_globalThis =
      typeof globalThis !== 'undefined'
        ? globalThis
        : typeof self !== 'undefined'
        ? self
        : typeof window !== 'undefined'
        ? window
        : typeof global !== 'undefined'
        ? global
        : {})
  )
}
//下次执行就直接返回 _globalThis，不需要第二次继续判断了 ！！！