自定义reducer,pipe,compose函数

Array的reducer方法;
//1:自定義reducer函數
Array.prototype.myReduce = function (
    fn = (pre, curr, index, arr) => {},
    initValue
) {
    let acc;
    for (let i = 0; i < this.length; i++) {
        if (initValue !== undefined && i === 0) {
            acc = initValue;
        }
        if (i === 0 && initValue === undefined) {
            acc = this[i];
        } else {
            acc = fn(acc, this[i], i);
        }
    }

    return acc;
};

// 測試1: without initValue
['a', 'b', 'c', 'd'].myReduce((pre, curr, i) => {
    console.log(
        `pre:${pre}, curr:${curr}, index:${i}, return value:${pre + curr}`
    );
    return pre + curr;
});

/* print result
        pre:a,    curr:b, index:1, return value:ab
        pre:ab,   curr:c, index:2, return value:abc
        pre:abc,  curr:d, index:3, return value:abcd
    */

// 測試2: with initValue
[('a', 'b', 'c', 'd')].myReduce((pre, curr, i) => {
    console.log(
        `pre:${pre}, curr:${curr}, index:${i}, return value:${pre + curr}`
    );
    return pre + curr;
}, 'jeff');

/* print result
    pre:jeff, curr:a, index:0, return value:jeffa
    pre:jeffa, curr:b, index:1, return value:jeffab
    pre:jeffab, curr:c, index:2, return value:jeffabc
    pre:jeffabc, curr:d, index:3, return value:jeffabcd 
*/

//2: 實現pipe管道函數
function a(s) {
    return s + 'a';
}

function b(s) {
    return s + 'b';
}
function upperCase(s) {
    return s.toUpperCase();
}
function addHello(s) {
    return s + ',hello';
}

[a, b].reduce((fx, fy) => {
    return function (arg) {
        return fy(fx(arg));
    };
})('jeff');

/**
 * 嵌套执行的函数平铺，從左到右 fy(fx(arg))
 * @param  {...any} funs fx(arg) fy(arg)
 * @returns function(arg){fy(fx(arg))}
 */
const pipe = (...funs) => {
    return function (...args) {
        const length = funs.length;
        if (length === 0) {
            return args;
        } else if (length === 1) {
            return funs[0](...args);
        }
        return funs.reduce((fx, fy) => {
            return fy(fx(...args));
        });
    };
};

const upperCaseThenAdd = pipe(upperCase, addHello);
upperCaseThenAdd('jeff');
// output:"JEFF,hello"

const addThenUpperCase = pipe(addHello, upperCase);
addThenUpperCase('jeff');
// output:"JEFF,HELLO"

const upperCaseString = pipe(upperCase);
upperCaseString('jeff');
// output:"JEFF"

//3: 實現compose函數
// 嵌套执行的函数平铺，從右到左

　　

自定义reducer,pipe,compose函数
Array的reducer方法;//1:自定義reducer函數Array.prototype.myReduce = function (    fn = (pre, curr, index, arr) => {},    initValue) {    let acc;    for (let i = 0; i < this.length; i++) {        if (initValue !== undefined && i === 0) {            acc = initValue;        }        if (i === 0 && initValue === undefined) {            acc = this[i];        } else {            acc = fn(acc, this[i], i);        }    }
    return acc;};
// 測試1: without initValue['a', 'b', 'c', 'd'].myReduce((pre, curr, i) => {    console.log(        `pre:${pre}, curr:${curr}, index:${i}, return value:${pre + curr}`    );    return pre + curr;});
/* print result        pre:a,    curr:b, index:1, return value:ab        pre:ab,   curr:c, index:2, return value:abc        pre:abc,  curr:d, index:3, return value:abcd    */
// 測試2: with initValue[('a', 'b', 'c', 'd')].myReduce((pre, curr, i) => {    console.log(        `pre:${pre}, curr:${curr}, index:${i}, return value:${pre + curr}`    );    return pre + curr;}, 'jeff');
/* print result    pre:jeff, curr:a, index:0, return value:jeffa    pre:jeffa, curr:b, index:1, return value:jeffab    pre:jeffab, curr:c, index:2, return value:jeffabc    pre:jeffabc, curr:d, index:3, return value:jeffabcd */
//2: 實現pipe管道函數function a(s) {    return s + 'a';}
function b(s) {    return s + 'b';}function upperCase(s) {    return s.toUpperCase();}function addHello(s) {    return s + ',hello';}
[a, b].reduce((fx, fy) => {    return function (arg) {        return fy(fx(arg));    };})('jeff');
/** * 嵌套执行的函数平铺，從左到右 fy(fx(arg)) * @param  {...any} funs fx(arg) fy(arg) * @returns function(arg){fy(fx(arg))} */const pipe = (...funs) => {    return function (...args) {        const length = funs.length;        if (length === 0) {            return args;        } else if (length === 1) {            return funs[0](...args);        }        return funs.reduce((fx, fy) => {            return fy(fx(...args));        });    };};
const upperCaseThenAdd = pipe(upperCase, addHello);upperCaseThenAdd('jeff');// output:"JEFF,hello"
const addThenUpperCase = pipe(addHello, upperCase);addThenUpperCase('jeff');// output:"JEFF,HELLO"
const upperCaseString = pipe(upperCase);upperCaseString('jeff');// output:"JEFF"
//3: 實現compose函數// 嵌套执行的函数平铺，從右到左