To execute a Python enumerate function in JavaScript


When developing a web application using Node.js, I was turning the list with forEach. I also needed index information in a hurry.

So I wrote the following source code, but my friends pointed out that there was a better way. When I actually tried it, it was very convenient and I felt that it was close to Python's enumerate function, so I will leave it in the article as a way to get the index and value at the same time with the ** for statement **.

ary = ["bar", "foo", "hoge"]

let i = 0 
ary.forEach(function( value ) {
     console.log(i, value );
// 0 bar
// 1 foo
// 2 hoge

Execution environment

Verification environment

What is the enumerate function in the first place?

A function that gets the index and value of the list specified in the argument.

ary = ["bar", "foo", "hoge"]
for i, item in enumerate(ary):
    print(i, item)
#  0 bar
#  1 foo
#  2 hoge

Get index and value at the same time with for statement

forEach can specify an index as the second argument

In the forEach function, the first argument is a list and the second argument is an index.

This index is automatically incremented as the number of loops increases, so You do not have to write i ++ or i + = 1.

ary = ["bar", "foo", "hoge"]
ary.forEach(function(value, i) {
     console.log(i, value);
// 0 bar
// 1 foo
// 2 hoge

Use for in

for in gets the index of the list. The index and value are retrieved by the following source code.

ary = ["bar", "foo", "hoge"]

for (i in ary){
    console.log(i, ary[i])
// 0 bar
// 1 foo
// 2 hoge

On the other hand, for in may show unexpected behavior as follows, so It seems that you need to be careful when using it.

var data = [ 'apple', 'orange', 'banana'];
//Added hoge method to array object
Array.prototype.hoge = function(){}
  for (var key in data){
// apple
// orange
// banana
// function(){}


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