Iterate through and access both array and object elements with the same code

Question

Let's say I have a function that receives either an object or an array. I would like to iterate through each element, and do something to each element as I go. I know that I can iterate through an array with forEach() or a normal for loop. I can also iterate through an object and array with a for in loop. But unlike the array case I have to check the object's elements with hasOwnProperty().

Can I just apply some general piece of code to both arrays and objects?

Update. This is my attempt for some object/array called value, but no console messages appear for the object case:

keysArray = Object.keys(value);
for(let key in keysArray) {
    console.log(value[key])
}

Perhaps [lodash collection methods](https://lodash.com/docs/4.17.5#map) are of use to you? — vapurrmaid, Mar 23 '18 at 02:34
Show some code that you understand we can take it from there. — zer00ne, Mar 23 '18 at 02:38
"*I have to check the object's elements with hasOwnProperty()*" - no, [you probably don't](https://stackoverflow.com/q/45014347/1048572). And no, you [shouldn't use `for…in` enumerations on arrays](https://stackoverflow.com/q/500504/1048572). — Bergi, Mar 23 '18 at 02:38

score 1 · Accepted Answer · answered Mar 23 '18 at 02:46

1

You could use

function* entries(o) {
    if (Array.isArray(o))
        for (let i=0; i<o.length; i++)
            yield [i, o[i]];
    else
        for (const p in o)
            yield [p, o[i]];
}
// or using builtin iterators:
function entries(o) {
    return Array.isArray(o) ? o.entries() : Object.entries(o).values();
}

and call it as

for (const [key, value] of entries(something)) {
    console.log(key, value);
}

Of course you can do a similar thing to get just the keys or just the values.

answered Mar 23 '18 at 02:46

Bergi

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Thank you @Bergi. I'll have to look this up. Does the `*` declare a generator? – cham Mar 23 '18 at 02:54
@chamilton Yes, that's a [generator function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*) – Bergi Mar 23 '18 at 03:05

score 1 · Answer 2 · answered Mar 23 '18 at 02:54

1

// whatever can be object or array

function doSth(whatever) {
  let myLoopable = whatever;

  // obj to array
  if (!Array.isArray(whatever)) {
    myLoopable = Object
      .keys(whatever) // get array of keys
      .reduce((acc, curKey) => [...acc, whatever[curKey]], []); // get array of values
  }

  myLoopable.forEach(value => console.log(value));
}

This is an example, not the best, you must do your own to check if it is object or not to make sure your function not failed

answered Mar 23 '18 at 02:54

Khoa

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I think you want `map`, not `reduce`. – Bergi Mar 23 '18 at 03:06
`map` is another choice :D. It's up to you – Khoa Mar 23 '18 at 03:07
Well it would be a great improvement in simplicity, readability and performance… – Bergi Mar 23 '18 at 03:11
for performance, I am sure I have no idea. for simplicity & readability, i am sure it equals. `map` and `reduce` have theirs own meaning and approach. I must agree that most people are not comfortable with `reduce`, but it's not because complexity. – Khoa Mar 23 '18 at 03:16
Yes, they have their own meaning - and when you mean to map the array to a new one, you should not use `reduce`. – Bergi Mar 23 '18 at 03:37
It would be nice if you could give me some references for why "you should not use reduce". Thanks – Khoa Mar 23 '18 at 03:42
I think I already gave you enough reasons. I can't provide more resources, you'll have to believe me. – Bergi Mar 23 '18 at 03:47
wow, you just say by yourself, no number, no chart, no information shows why "should NOT use `reduce` when mapping an array to a new one", or how fast to use `map` over `reduce`. Ok, if you don't want to share knowledge, ok, fine. – Khoa Mar 23 '18 at 04:00
What chart, what number do you need to judge simplicity and readability? Apply a code metric you believe in. If you want to compare performance, do a benchmark - I don't need to do it for you. – Bergi Mar 23 '18 at 04:49
Have you tried writing a solution with `map`, do you know what it would look like? Please post it. The comparison should be obvious then. – Bergi Mar 23 '18 at 04:50
Ok, end here, you gave a statement, I asked for an evidence. You didn't want to give it. I also don't have time and duty to prove what you said. You even didn't read my question. I've never mention about simplicity and readability in my question. – Khoa Mar 23 '18 at 07:59

Iterate through and access both array and object elements with the same code

2 Answers2