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// ES5循环循环实现filter

const selfFilter = function (fn, context) {

let arr = Array.prototype.slice.call(this)

let filteredArr = []

for (let i = 0; i < arr.length; i++) {

if(!arr.hasOwnProperty(i)) continue;

fn.call(context, arr[i], i, this) && filteredArr.push(arr[i])

}

return filteredArr

}

// reduce实现filter

const selfFilter2 = function (fn, context) {

return this.reduce((pre, cur, index) => {

return fn.call(context, cur, index, this) ? [...pre, cur] : [...pre]

}, [])

}

Array.prototype.selfFilter || (Object.defineProperty(Array.prototype, 'selfFilter', {

value: selfFilter,

enumerable: false,

configurable: true,

writable: true

}))

Array.prototype.selfFilter2 || (Object.defineProperty(Array.prototype, 'selfFilter2', {

value: selfFilter2,

enumerable: false,

configurable: true,

writable: true

}))

let arr = [1, 2, 3]

console.log(arr.selfFilter(item => item === 2))

//当给filter函数传入第二个参数时,第一个参数不能为箭头函数,否则为词法绑定，第二个参数会失效，其他迭代方法同理

console.log(arr.selfFilter2(function (item) {

return item === 2

}, ['a', 'b', 'c']))