What Is the Es6 Equivalent of Python 'Enumerate' for a Sequence?

Array.prototype.map

Array.prototype.map already gives you the index as the second argument to the callback procedure... And it's supported almost everywhere.

['a','b'].map(function(element, index) { return index + ':' + element; });
//=> ["0:a", "1:b"]

I like ES6 too

['a','b'].map((e,i) => `${i}:${e}`)
//=> ["0:a", "1:b"]

make it lazy

However, python's enumerate is lazy and so we should model that characteristic as well -

function* enumerate (it, start = 0)
{ let i = start
  for (const x of it)
    yield [i++, x]
}

for (const [i, x] of enumerate("abcd"))
  console.log(i, x)

0 a
1 b
2 c
3 d

Specifying the second argument, start, allows the caller to control the transform of the index -

for (const [i, x] of enumerate("abcd", 100))
  console.log(i, x)

100 a
101 b
102 c
103 d

Excuse me if I'm being ignorant (bit of a newbie to JavaScript here), but can't you just use forEach? e.g:

function withIndex(elements) {
    var results = [];
    elements.forEach(function(e, ind) {
        results.push(`${e}:${ind}`);
    });
    return results;
}

alert(withIndex(['a', 'b']));

There's also naomik's answer which is a better fit for this particular use case, but I just wanted to point out that forEach also fits the bill.

ES5+ supported.

pythonic offers an enumerate function that works on all iterables, not just arrays, and returns an Iterator, like python:

import {enumerate} from 'pythonic';

const arr = ['a', 'b'];
for (const [index, value] of enumerate(arr))
    console.log(`index: ${index}, value: ${value}`);
// index: 0, value: a
// index: 1, value: b

Disclosure I'm author and maintainer of Pythonic

as Kyle and Shanoor say is Array.prototype.entries()

but for newbie like me, hard to fully understand its meaning.

so here give an understandable example:

for(let curIndexValueList of someArray.entries()){
  console.log("curIndexValueList=", curIndexValueList)
  let curIndex = curIndexValueList[0]
  let curValue = curIndexValueList[1]
  console.log("curIndex=", curIndex, ", curValue=", curValue)
}

equivalent to python code:

for curIndex, curValue in enumerate(someArray):
  print("curIndex=%s, curValue=%s" % (curIndex, curValue))
}