JS数组中map,filter,reduce实现

1.map的实现

function map(arr, cb) {
    if (!Array.isArray(arr) || !arr.length || typeof cb !== 'function') {
        return [];
    }
    const result = [];
    for (let i = 0; i < arr.length; i++) {
        result.push(cb(arr[i], i, arr));
    }
    return result;
}

Array.prototype.map=function(cb){
    return map(this,cb);
}
const result=[1,2,3,4].map((item,index)=>item>3);
console.log(result)

2.filter的实现

function filter(arr, cb) {
    if (!Array.isArray(arr) || !arr.length || typeof cb !== 'function') {
        return [];
    }
    const result = [];
    for (let i = 0; i < arr.length; i++) {
        cb(arr[i], i, arr)&&result.push(arr[i]);
    }
    return result;
}

Array.prototype.filter = function (cb) {
    return filter(this, cb);
}

const result = [1, 2, 3, 4, 5].filter(item => item > 4);
console.log(result)

3.reduce的实现

function reduce(arr, initialValue, cb) {
    if (!Array.isArray(arr) || !arr.length || typeof cb !== 'function') {
        return [];
    }
    const hasInitialValue = initialValue !== undefined;
    let value = hasInitialValue ? initialValue : arr[0];
    for (let i = hasInitialValue ? 0 : 1; i < arr.length; i++) {
        value = cb(value, arr[i], i, arr);
    }
    return value;
}

Array.prototype.reduce = function (cb, initialValue) {
    return reduce(this, initialValue, cb)
}

const result = [1, 2, 3, 4, 5].reduce((memo, current, index) => {
    memo.push(current + 1)
    return memo;
}, []);
console.log(result)