Array.map：获取要在返回函数中使用的每个元素的索引

Question

I'm looking for a command that returns the index of an element so I can use it in map functions.

Here's an example :

function myFunction() {
  var a = [4,7,9];
  //Looking for a way to return ["index 0 : 4", "index 1 : 7", "index 2 : 9"]
  //Only workaround found so far :
  var i = -1;
  Logger.log(a.map(function(el) {i++; return "index " + i + " : " + el}));
}

I'm sure there's a neater way with a command that would give me the element's index, but all my google searches have been fruitless so far.

Answer 1

Second parameter in callback function is index you can use that

  Array.map((value,index,this))

 function myFunction() { var a = [4,7,9]; console.log(a.map((v,i)=> "index " + i + " : " + v)); } myFunction() 

Answer 2

You can make it cleaner by three steps:

• Use the second parameter of callback passed to map() which is index of element.
• Use arrow function and implicitly return the value
• Use Template Strings instead of + again and again.

 var a = [4,7,9]; let res = a.map((x,i) => `index ${i} : ${x}`); console.log(res) 

In your above code you have created a function which doesn't make any sense. That function should take an array as a parameter and then log the value of that particular arr.

 const myFunction = arr => console.log(arr.map((x,i) => `index ${i} : ${x}`)); myFunction([4,7,9]) 

Answer 3

map already gives you the index. The callback gets three parameters:

a.map(function(element, index, array) {return "index " + index + " : " + element})

You don't need to declare all three (JavaScript isn't fussy about it), but they are there if you need them.

Answer 4

You could also simply use Array.from since its second argument is an Array.map :

 console.log(Array.from([4,7,9], (x, i) => `index ${i} : ${x}`)) 

The overall idea is to use the 2nd parameter of map (which provides you with the current iteration index) and with that and the current value to construct your result.