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])
}

Show some code that you understand we can take it from there. — zer00ne
– zer00ne, Commented Mar 23, 2018 at 2:38
"I have to check the object's elements with hasOwnProperty()" - no, you probably don't. And no, you shouldn't use for…in enumerations on arrays. — Bergi
– Bergi, Commented Mar 23, 2018 at 2:38

Bergi · Accepted Answer · 2018-03-23 02:46:14Z

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, 2018 at 2:46

Bergi

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2 Comments

cham Over a year ago

Thank you @Bergi. I'll have to look this up. Does the * declare a generator?

Bergi Over a year ago

@chamilton Yes, that's a generator function

Khoa · Accepted Answer · 2018-03-23 02:54:45Z

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, 2018 at 2:54

Khoa

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11 Comments

Bergi Over a year ago

I think you want map, not reduce.

Khoa Over a year ago

map is another choice :D. It's up to you

Bergi Over a year ago

Well it would be a great improvement in simplicity, readability and performance…

Khoa Over a year ago

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.

Bergi Over a year ago

Yes, they have their own meaning - and when you mean to map the array to a new one, you should not use reduce.

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