lodash.js源码笔记1

源码学习笔记将按照lodash源码js文件的字母顺序书写

add(number1, number2), 接收两个number类型参数，非常简单，就是做加法(求和)处理，返回“和”

add.js 源码：

import createMathOperation from './.internal/createMathOperation.js'

/**
 * Adds two numbers.
 *
 * @since 3.4.0
 * @category Math
 * @param {number} augend The first number in an addition.
 * @param {number} addend The second number in an addition.
 * @returns {number} Returns the total.
 * @example
 *
 * add(6, 4)
 * // => 10
 */
const add = createMathOperation((augend, addend) => augend + addend, 0)
// add方法的实现



export default add

add.js主要是引用了createMathOperation方法, 下面贴出createMathOperation.js完整源码

createMathOperation.js 源码：

import baseToNumber from './baseToNumber.js'
import baseToString from './baseToString.js'

/**
 * Creates a function that performs a mathematical operation on two values.
 * // 创建一个对两个值执行数学运算的函数 
 *
 * @private
 * @param {Function} operator The function to perform the operation. // 参数，执行函数
 * @param {number} [defaultValue] The value used for `undefined` arguments. // 默认值，当未传递参数时返回
 * @returns {Function} Returns the new mathematical operation function. // 返回一个函数
 */
function createMathOperation(operator, defaultValue) {
  // value 、 other 作为箭头函数的参数如何获取的呢
  return (value, other) => {
    if (value === undefined && other === undefined) {
      return defaultValue
    }
    if (value !== undefined && other === undefined) {
      return value
    }
    if (other !== undefined && value === undefined) {
      return other
    }
    if (typeof value == 'string' || typeof other == 'string') {
      value = baseToString(value)
      other = baseToString(other)
    }
    else {
      value = baseToNumber(value)
      other = baseToNumber(other)
    }
    return operator(value, other)
  }
}

export default createMathOperation

baseToNumber.js 源码：

import isSymbol from '../isSymbol.js'

/** Used as references for various `Number` constants. */
const NAN = 0 / 0

/**
 * The base implementation of `toNumber` which doesn't ensure correct
 * conversions of binary, hexadecimal, or octal string values.
 *
 * @private
 * @param {*} value The value to process.
 * @returns {number} Returns the number.
 */
function baseToNumber(value) {
  if (typeof value == 'number') {
    return value
  }
  if (isSymbol(value)) {
    return NAN
  }
  return +value
}

export default baseToNumber

baseToString.js 源码：

import isSymbol from '../isSymbol.js'

/** Used as references for various `Number` constants. */
const INFINITY = 1 / 0

/** Used to convert symbols to primitives and strings. */
const symbolProto = Symbol ? Symbol.prototype : undefined
const symbolToString = symbolProto ? symbolProto.toString : undefined

/**
 * The base implementation of `toString` which doesn't convert nullish
 * values to empty strings.
 *
 * @private
 * @param {*} value The value to process.
 * @returns {string} Returns the string.
 */
function baseToString(value) {
  // Exit early for strings to avoid a performance hit in some environments.
  if (typeof value == 'string') {
    return value
  }
  if (Array.isArray(value)) {
    // Recursively convert values (susceptible to call stack limits).
    return `${value.map(baseToString)}`
  }
  if (isSymbol(value)) {
    return symbolToString ? symbolToString.call(value) : ''
  }
  const result = `${value}`
  return (result == '0' && (1 / value) == -INFINITY) ? '-0' : result
}

export default baseToString

isSymbol.js 源码：

import getTag from './.internal/getTag.js'

/**
 * Checks if `value` is classified as a `Symbol` primitive or object.
 *
 * @since 4.0.0
 * @category Lang
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is a symbol, else `false`.
 * @example
 *
 * isSymbol(Symbol.iterator)
 * // => true
 *
 * isSymbol('abc')
 * // => false
 */
function isSymbol(value) {
  const type = typeof value
  return type == 'symbol' || (type == 'object' && value != null && getTag(value) == '[object Symbol]')
}

export default isSymbol

getTag.js 源码：

import baseGetTag from './baseGetTag.js'

/** `Object#toString` result references. */
const dataViewTag = '[object DataView]'
const mapTag = '[object Map]'
const objectTag = '[object Object]'
const promiseTag = '[object Promise]'
const setTag = '[object Set]'
const weakMapTag = '[object WeakMap]'

/** Used to detect maps, sets, and weakmaps. */
const dataViewCtorString = `${DataView}`
const mapCtorString = `${Map}`
const promiseCtorString = `${Promise}`
const setCtorString = `${Set}`
const weakMapCtorString = `${WeakMap}`

/**
 * Gets the `toStringTag` of `value`.
 *
 * @private
 * @param {*} value The value to query.
 * @returns {string} Returns the `toStringTag`.
 */
let getTag = baseGetTag

// Fallback for data views, maps, sets, and weak maps in IE 11 and promises in Node.js < 6.
if ((DataView && getTag(new DataView(new ArrayBuffer(1))) != dataViewTag) ||
    (getTag(new Map) != mapTag) ||
    (getTag(Promise.resolve()) != promiseTag) ||
    (getTag(new Set) != setTag) ||
    (getTag(new WeakMap) != weakMapTag)) {
  getTag = (value) => {
    const result = baseGetTag(value)
    const Ctor = result == objectTag ? value.constructor : undefined
    const ctorString = Ctor ? `${Ctor}` : ''

    if (ctorString) {
      switch (ctorString) {
        case dataViewCtorString: return dataViewTag
        case mapCtorString: return mapTag
        case promiseCtorString: return promiseTag
        case setCtorString: return setTag
        case weakMapCtorString: return weakMapTag
      }
    }
    return result
  }
}

export default getTag

baseGetTag.js 源码：

const objectProto = Object.prototype
const hasOwnProperty = objectProto.hasOwnProperty
const toString = objectProto.toString
const symToStringTag = typeof Symbol != 'undefined' ? Symbol.toStringTag : undefined

/**
 * The base implementation of `getTag` without fallbacks for buggy environments.
 *
 * @private
 * @param {*} value The value to query.
 * @returns {string} Returns the `toStringTag`.
 */
function baseGetTag(value) {
  if (value == null) {
    return value === undefined ? '[object Undefined]' : '[object Null]'
  }
  if (!(symToStringTag && symToStringTag in Object(value))) {
    return toString.call(value)
  }
  const isOwn = hasOwnProperty.call(value, symToStringTag)
  const tag = value[symToStringTag]
  let unmasked = false
  try {
    value[symToStringTag] = undefined
    unmasked = true
  } catch (e) {}

  const result = toString.call(value)
  if (unmasked) {
    if (isOwn) {
      value[symToStringTag] = tag
    } else {
      delete value[symToStringTag]
    }
  }
  return result
}

export default baseGetTag