实现数组方法

用于熟悉数组API底层原理

map()

• 原生实现 map

  Array.prototype.myMap = function (fn, thisValue) {
    if (!Array.isArray(this) || typeof fn !== 'function') {
      throw new Error('has error')
    }
    let result = []
    thisValue = thisValue || globalThis
    let sourceArr = this
  
    for (let i = 0; i < sourceArr.length; i++) {
      result.push(fn.call(thisValue, sourceArr[i], i, sourceArr))
    }
    return result
  }
  

• 使用 Array 的 reduce 方法

  Array.prototype.myMapOfReduce = function (fn, thisValue) {
    if (!Array.isArray(this) || typeof fn !== 'function') {
      throw new Error('has error')
    }
    const _this = this
    const result = []
    _this.reduce((pre, current, currentIndex, _this) => {
      result.push(fn.call(thisValue, current, currentIndex, this))
    }, undefined)
    return result
  }
  

reduce()

• 原生

  Array.prototype.myReduce = function (fn, initValue) {
    const sourceArr = this
    const len = sourceArr.length
    if (!Array.isArray(sourceArr) || typeof fn !== 'function') {
      throw new Error('has error')
    }
    if (len === 0 && initValue === undefined) {
      throw new Error('Reduce of empty array with no initial value')
    }
    let i = initValue === undefined ? 1 : 0
    initValue = initValue === undefined ? sourceArr[0] : initValue
    for (i; i < len; i++) {
      initValue = fn(initValue, sourceArr[i], i, sourceArr)
    }
    return initValue
  }