收下了，Vue 2源码中6个实用的工具函数

1. 确定值的类型

/**
 * Get the raw type string of a value, e.g., [object Object].
 */
const _toString = Object.prototype.toString

export function toRawType (value) {
  return _toString.call(value).slice(8, -1)
}

2. 获得一个值（含对象、数组）的字符串表示

/**
 * Convert a value to a string that is actually rendered.
 */
const _toString = Object.prototype.toString
 
export function toString (val) {
  return val == null
    ? ''
    : Array.isArray(val) || (isPlainObject(val) && val.toString === _toString)
      ? JSON.stringify(val, null, 2)
      : String(val)
}

3. 在函数里加入缓存控制

/**
 * Create a cached version of a pure function.
 */
export function cached(fn) {
  const cache = Object.create(null)
  return (function cachedFn (str) {
    const hit = cache[str]
    return hit || (cache[str] = fn(str))
  })
}

4. 判断两个值是否宽松相等——如果两个值都是对象且它们‘模样’完全一致，则它们宽松相等

/**
 * Check if two values are loosely equal - that is,
 * if they are plain objects, do they have the same shape?
 */
export function looseEqual (a, b) {
  if (a === b) return true
  const isObjectA = isObject(a)
  const isObjectB = isObject(b)
  if (isObjectA && isObjectB) {
    try {
      const isArrayA = Array.isArray(a)
      const isArrayB = Array.isArray(b)
      if (isArrayA && isArrayB) {
        return a.length === b.length && a.every((e, i) => {
          return looseEqual(e, b[i])
        })
      } else if (a instanceof Date && b instanceof Date) {
        return a.getTime() === b.getTime()
      } else if (!isArrayA && !isArrayB) {
        const keysA = Object.keys(a)
        const keysB = Object.keys(b)
        return keysA.length === keysB.length && keysA.every(key => {
          return looseEqual(a[key], b[key])
        })
      } else {
        return false
      }
    } catch (e) {
      return false
    }
  } else if (!isObjectA && !isObjectB) {
    return String(a) === String(b)
  } else {
    return false
  }
}

5. 确保某个函数只执行一次

/**
 * Ensure a function is called only once.
 */
export function once (fn) {
  let called = false
  return function () {
    if (!called) {
      called = true
      fn.apply(this, arguments)
    }
  }
}

6. 判断浏览器是否原生支持某个特性

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