map()函数内的索引

Question

我错过了一个选项如何map使用Listfrom 获取函数内的索引号Immutable.js:

var list2 = list1.map(mapper => { a: mapper.a, b: mapper.index??? }).toList();

文档显示的是map()回报Iterable<number, M>.有什么优雅的方式来满足我的需求吗？

Answer 1

你将能够获得当前迭代index的map通过它的第二个参数的API.

另请参阅: https ://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/map

参数

callback - 生成新数组元素的函数,带有三个参数:

1)currentValue
数组中正在处理的当前元素.

2)index
数组中正在处理的当前元素的索引.

3)数组
调用了数组映射.

例:

const list = [ 'h', 'e', 'l', 'l', 'o'];
list.map((currElement, index) => {
  console.log("The current iteration is: " + index);
  console.log("The current element is: " + currElement);
  console.log("\n");
  return currElement; //equivalent to list[index]
});

输出:

当前迭代为:0
当前元素为:h

当前迭代是:1
当前元素是:e

当前迭代是:2
当前元素是:l

当前迭代是:3
当前元素是:l

当前迭代是:4
当前元素是:o

Answer 2

Array.prototype.map() 指数:

可以Array.prototype.map()通过回调函数的第二个参数访问索引.这是一个例子:

const array = [1, 2, 3, 4];


const map = array.map((x, index) => {
  console.log(index);
  return x + index;
});

console.log(map);

其他论点Array.prototype.map():

• 回调函数的第三个参数公开调用映射的数组
• 第二个参数Array.map()是一个对象,它将是this回调函数的值.请记住,您必须使用regular function关键字才能声明回调,因为箭头函数没有自己的this关键字绑定.

例如:

const array = [1, 2, 3, 4];

const thisObj = {prop1: 1}


const map = array.map( function (x, index, array) {
  console.log(array);
  console.log(this)
}, thisObj);

Answer 3

• 假设你有一个像这样的数组

   const arr = [1, 2, 3, 4, 5, 6, 7, 8, 9]
    
    
    arr.map((myArr, index) => {
      console.log(`your index is -> ${index} AND value is ${myArr}`);
    })

> output will be
 index is -> 0 AND value is 1
 index is -> 1 AND value is 2
 index is -> 2 AND value is 3
 index is -> 3 AND value is 4
 index is -> 4 AND value is 5
 index is -> 5 AND value is 6
 index is -> 6 AND value is 7
 index is -> 7 AND value is 8
 index is -> 8 AND value is 9