JS篇--Higher Order Functions & Callbacks

今天主要学习了对于higher order function及callbacks的理解，记录下自己的学习体会，以便日后复习。

1. 什么是higher order function

中文翻译成高阶函数，在数学和计算机科学中，高阶函数是至少具有下列功能之一的函数:

1. Takes in a function or passes out a function
2. Just a term to describe these functions - any function that does it we call that - but
   there’s nothing different about them inherently

换句话说，可以简单理解成“the outer function that takes in a little baby function, or by the way returns out a function.

1. 将一个或多个函数作为参数(即过程参数)。
2. 返回一个函数作为结果。

2. 什么是Callbacks

What is callbakcs? Here is an explaination: The function we insert in is our callback function.

3. Examples

概念比较抽象，接下来用具体的例子来解释以下：

function copyArrayAndManipulate(array, instructions) {
 const output = [];
 for (let i = 0; i < array.length; i++) {
 output.push(instructions(array[i]));
 }
 return output;
}

function mutiplyByTwo(input){
  return input*2;
}

const newArray =[1,2,3]
const result = copyArrayAndManipulate(newArray,mutiplyByTwo);

在上面这个简单计算一串array的二倍数并返回新array的代码中，我们可以看到:

1. Higher order function: copyArrayAndManipulate function
2. callbacks: multipleByTwo function

4. Appendix

在这里，顺便介绍以下arrow function, 以更简单的表达方式来写function

function multiplyBy2(input) { return input * 2; }

⇊

const multiplyBy2 = (input) => { return input*2 }

⇊

const multiplyBy2 = (input) => input*2

⇊

const multiplyBy2 = input => input*2

⇊

const output = multiplyBy2(3) //6

以上的function写法所得到的结果都是相同的

remember: All the designers of JavaScript love to reduce code. This is maybe for legibility, but maybe not for readability.

5. 练习实操

1. Construct a function intersection that compares input arrays and returns a new array with elements found in all of the inputs. BONUS: Use reduce!（求交集）

Your Answer Here

思路：
2. 首先实现能在两个array找到相同的数；
3. 再实现在多个数组中找到交集；

Answer

const intersection = (xs, ys) => xs.filter(x => ys.indexOf(x) > -1)

const intersectAll = (...xss) => xss.reduce(intersection);

intersectAll([5, 10, 15, 20, 3], [15, 88, 3, 1, 5, 7],  [1, 10, 15, 5, 20,6], [3,4,6,7,5,15])
//[5,15]

2. 求并集

Your answer Here

Answer:

function array_remove_repeat(array){
  let newArray =[];
   for(let i=0; i<array.length;i++){
     let temp = array[i];
     let flag = true;
     for(let j=0;j<newArray.length;j++){
       if(temp == newArray[j]){
         flag =false;
         break
       } }
       if(flag){
         newArray.push(temp)
     }  
  }
 return newArray;
}

function array_union(a, b) { // 并集
    return array_remove_repeat(a.concat(b));
}

const unionAll = (...xss) => xss.reduce(array_union);

unionAll([1,2,3,4],[56,7,3],[1,23,5])
//[1,  2,  3, 4, 56,  7, 23,  5]