Vue 源码解析之工具方法

const emptyObject = Object.freeze({})

function isUndef (v) {
  return v === undefined || v === null
}

function isDef (v) {
  return v !== undefined && v !== null
}

function isTrue (v) {
  return v === true
}

function isFalse (v) {
  return v === false
}

function isPrimitive (value) {
  return (
    typeof value === 'string' ||
    typeof value === 'number' ||
    // $flow-disable-line
    typeof value === 'symbol' ||
    typeof value === 'boolean'
  )
}

export function isObject (obj) {
  return obj !== null && typeof obj === 'object'
}

const _toString = Object.prototype.toString

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

export function isPlainObject (obj) {
  return _toString.call(obj) === '[object Object]'
}

export function isValidArrayIndex (val) {
  const n = parseFloat(String(val))
  return n >= 0 && Math.floor(n) === n && isFinite(val)
}

export function isPromise (val) {
  return (
    isDef(val) &&
    typeof val.then === 'function' &&
    typeof val.catch === 'function'
  )
}

export function toString (val) {
  return val == null
    ? ''
    : Array.isArray(val) || (isPlainObject(val) && val.toString === _toString)
      ? JSON.stringify(val, null, 2)
      : String(val)
}

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

export function makeMap (
  str,
  expectsLowerCase
) {
  const map = Object.create(null)
  const list: Array<string> = str.split(',')
  for (let i = 0; i < list.length; i++) {
    map[list[i]] = true
  }
  return expectsLowerCase
    ? val => map[val.toLowerCase()]
    : val => map[val]
}

export const isBuiltInTag = makeMap('slot,component', true)

export const isReservedAttribute = makeMap('key,ref,slot,slot-scope,is')

export function remove (arr, item) {
  if (arr.length) {
    const index = arr.indexOf(item)
    if (index > -1) {
      return arr.splice(index, 1)
    }
  }
}

const hasOwnProperty = Object.prototype.hasOwnProperty
export function hasOwn (obj, key) {
  return hasOwnProperty.call(obj, key)
}

export function cached(fn) {
  const cache = Object.create(null)
  return (function cachedFn (str) {
    const hit = cache[str]
    return hit || (cache[str] = fn(str))
  }: any)
}

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

export const capitalize = cached((str) => {
  return str.charAt(0).toUpperCase() + str.slice(1)
})

const hyphenateRE = /\B([A-Z])/g
export const hyphenate = cached((str) => {
  return str.replace(hyphenateRE, '-$1').toLowerCase()
})

function polyfillBind (fn, ctx) {
  function boundFn (a) {
    const l = arguments.length
    return l
      ? l > 1
        ? fn.apply(ctx, arguments)
        : fn.call(ctx, a)
      : fn.call(ctx)
  }

  boundFn._length = fn.length
  return boundFn
}

function nativeBind (fn, ctx) {
  return fn.bind(ctx)
}

export const bind = Function.prototype.bind
  ? nativeBind
  : polyfillBind

export function toArray (list, start) {
  start = start || 0
  let i = list.length - start
  const ret: Array<any> = new Array(i)
  while (i--) {
    ret[i] = list[i + start]
  }
  return ret
}

export function extend (to, _from) {
  for (const key in _from) {
    to[key] = _from[key]
  }
  return to
}

export function toObject (arr) {
  const res = {}
  for (let i = 0; i < arr.length; i++) {
    if (arr[i]) {
      extend(res, arr[i])
    }
  }
  return res
}

export function noop (a, b, c) {}

/**
 * Always return false.
 */
export const no = (a, b, c) => false

/**
 * Return the same value.
 */
export const identity = (_) => _

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