Vue2.6源码shared/utils.js

export const emptyObject = Object.freeze({})
export function isUndef (v) {
  return v === undefined || v === null
}
export function isDef (v) {
  return v !== undefined && v !== null
}

export function isTrue (v) {
  return v === true
}
export function isFalse (v) {
  return v === false
}

export function isPrimitive (value) {
  /* 原始值 */
  return (
    typeof value === 'number' ||
    typeof value === 'boolean' ||
    typeof value === 'symbol' ||
    typeof value === 'string'
  )
}

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

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

export function toRawType (value) {
  return _toString.call(value).slice(8, -1)
}
export function isRegExp (v) {
  return _toString.call(v) === '[object RegExp]'
}

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 = 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))
  })
}

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)
})

function polyfillBind(fn, ctx) {
  function boundFn(a) {
    const v = arguments.length
    return v
      ? v > 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 = 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?) { }
export const no = (a?, b?, c?) => false
export const identity = (_) => _
export function genStaticKeys(modules) {
  return modules.reduce((keys, m) => {
    return keys.concat(m.staticKeys || [])
  }, []).join(',')
}

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 {
        /* istanbul ignore next */
        return false
      }
    } catch (e) {
      /* istanbul ignore next */
      return false
    }
  } else if (!isObjectA && !isObjectB) {
    return String(a) === String(b)
  } else {
    return false
  }
}

export function looseIndexOf(arr,val) {
  for (let i = 0; i < arr.length; i++) {
    if (looseEqual(arr[i], val)) return i
  }
  return -1
}
export function once(fn) {
  let called = false
  return function () {
    if (!called) {
      called = true
      fn.apply(this, arguments)
    }
  }
}

true || false && false ==> true
true || false && true ==> true
true || true && true ==> true
true || true && false ==> true
false || false && false ==> false
false || false && true ==> false
false || true && true ==> true
false || true && false ==> false