util.js

/* @flow */

// these helpers produces better vm code in JS engines due to their
// explicitness and function inlining
export function isUndef (v: any): boolean %checks {
  return v === undefined || v === null
}

export function isDef (v: any): boolean %checks {
  return v !== undefined && v !== null
}

export function isTrue (v: any): boolean %checks {
  return v === true
}

/**
 * Check if value is primitive
 */
export function isPrimitive (value: any): boolean %checks {
  return typeof value === 'string' || typeof value === 'number'
}

/**
 * Quick object check - this is primarily used to tell
 * Objects from primitive values when we know the value
 * is a JSON-compliant type.
 */
export function isObject (obj: mixed): boolean %checks {
  return obj !== null && typeof obj === 'object'
}

const _toString = Object.prototype.toString

/**
 * Strict object type check. Only returns true
 * for plain JavaScript objects.
 */
 /*对对象类型进行严格检查，只有当对象是纯javascript对象的时候返回true*/
export function isPlainObject (obj: any): boolean {
  return _toString.call(obj) === '[object Object]'
}

export function isRegExp (v: any): boolean {
  return _toString.call(v) === '[object RegExp]'
}

/**
 * Convert a value to a string that is actually rendered.
 */
 /*将val转化成字符串*/
export function toString (val: any): string {
  return val == null
    ? ''
    : typeof val === 'object'
      ? JSON.stringify(val, null, 2)
      : String(val)
}

/**
 * Convert a input value to a number for persistence.
 * If the conversion fails, return original string.
 */
 /*将字符串转化为数字，如果转换失败会返回原字符串*/
export function toNumber (val: string): number | string {
  const n = parseFloat(val)
  return isNaN(n) ? val : n
}

/**
 * Make a map and return a function for checking if a key
 * is in that map.
 */
 /*
返回一个函数用以检测是否一个key值存在这个函数中
比如str = "a, b, c"
则返回 (key) => {
  return map[key];
}
map为{
  a: true,
  b: true,
  c: true
}
存在expectsLowerCase参数的时候会将所有的参数转化成小写
 */
}
}
export function makeMap (
  str: string,
  expectsLowerCase?: boolean
): (key: string) => true | void {
  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]
}

/**
 * Check if a tag is a built-in tag.
 */
export const isBuiltInTag = makeMap('slot,component', true)

/**
 * Remove an item from an array
 */
export function remove (arr: Array<any>, item: any): Array<any> | void {
  if (arr.length) {
    const index = arr.indexOf(item)
    if (index > -1) {
      return arr.splice(index, 1)
    }
  }
}

/**
 * Check whether the object has the property.
 */
const hasOwnProperty = Object.prototype.hasOwnProperty
export function hasOwn (obj: Object | Array<*>, key: string): boolean {
  return hasOwnProperty.call(obj, key)
}

/**
 * Create a cached version of a pure function.
 */
 /*根据str得到fn(str)的结果，但是这个结果会被闭包中的cache缓存起来，下一次如果是同样的str则不需要经过fn(str)重新计算，而是直接得到结果*/
export function cached<F: Function> (fn: F): F {
  const cache = Object.create(null)
  return (function cachedFn (str: string) {
    const hit = cache[str]
    return hit || (cache[str] = fn(str))
  }: any)
}

/**
 * Camelize a hyphen-delimited string.
 */
 /*将原本用-连接的字符串变成驼峰 aaa-bbb-ccc => aaaBbbCcc*/
const camelizeRE = /-(\w)/g
export const camelize = cached((str: string): string => {
  return str.replace(camelizeRE, (_, c) => c ? c.toUpperCase() : '')
})

/**
 * Capitalize a string.
 */
 /*首字母转大写*/
export const capitalize = cached((str: string): string => {
  return str.charAt(0).toUpperCase() + str.slice(1)
})

/**
 * Hyphenate a camelCase string.
 */
 /*连接一个camelCase字符串。*/
const hyphenateRE = /([^-])([A-Z])/g
export const hyphenate = cached((str: string): string => {
  return str
    .replace(hyphenateRE, '$1-$2')
    .replace(hyphenateRE, '$1-$2')
    .toLowerCase()
})

/**
 * Simple bind, faster than native
 */
export function bind (fn: Function, ctx: Object): Function {
  function boundFn (a) {
    const l: number = arguments.length
    return l
      ? l > 1
        ? fn.apply(ctx, arguments)
        : fn.call(ctx, a)
      : fn.call(ctx)
  }
  // record original fn length
  boundFn._length = fn.length
  return boundFn
}

/**
 * Convert an Array-like object to a real Array.
 */
 /*将类数组的对象转换成数组*/
export function toArray (list: any, start?: number): Array<any> {
  start = start || 0
  let i = list.length - start
  const ret: Array<any> = new Array(i)
  while (i--) {
    ret[i] = list[i + start]
  }
  return ret
}

/**
 * Mix properties into target object.
 */
 /*将_from的属性混合（会覆盖）to对象中*/
export function extend (to: Object, _from: ?Object): Object {
  for (const key in _from) {
    to[key] = _from[key]
  }
  return to
}

/**
 * Merge an Array of Objects into a single Object.
 */
 /*合并Array数组中的每一个对象到一个新的Object中*/
export function toObject (arr: Array<any>): Object {
  const res = {}
  for (let i = 0; i < arr.length; i++) {
    if (arr[i]) {
      extend(res, arr[i])
    }
  }
  return res
}

/**
 * Perform no operation.
 */
export function noop () {}

/**
 * Always return false.
 */
export const no = () => false

/**
 * Return same value
 */
export const identity = (_: any) => _

/**
 * Generate a static keys string from compiler modules.
 */
export function genStaticKeys (modules: Array<ModuleOptions>): string {
  return modules.reduce((keys, m) => {
    return keys.concat(m.staticKeys || [])
  }, []).join(',')
}

/**
 * Check if two values are loosely equal - that is,
 * if they are plain objects, do they have the same shape?
 */
 /*检测两个变量是否相等*/
export function looseEqual (a: mixed, b: mixed): boolean {
  const isObjectA = isObject(a)
  const isObjectB = isObject(b)
  if (isObjectA && isObjectB) {
    try {
      return JSON.stringify(a) === JSON.stringify(b)
    } catch (e) {
      // possible circular reference
      return a === b
    }
  } else if (!isObjectA && !isObjectB) {
    return String(a) === String(b)
  } else {
    return false
  }
}

/*检测arr数组中是否包含与val变量相等的项*/
export function looseIndexOf (arr: Array<mixed>, val: mixed): number {
  for (let i = 0; i < arr.length; i++) {
    if (looseEqual(arr[i], val)) return i
  }
  return -1
}

/**
 * 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)
    }
  }
}