How to get only properties (not values) of an array?

From the answer of Why does a string index in an array not increase the 'length'?, I found out that arrays can have both value and properties.

For example in the following code:

var array = [];
array['test1'] = 'property 1';
array['test2'] = 'property 1';
array[0] = 'value 1';
array[1] = 'value 2';

Second and third line create property.

Fourth and fifth line create value.

I can get the values using:

array.forEach((value) => {
    console.log(value);
});

How can I get only the properties?

> Second and third line create property.
>
> Fourth and fifth line create value.

No. All of those lines create a property and assigns a value to it.

The only difference is that some of those property names are integers.

> How can I get only the properties?

If you want to use properties that are not numbers then do not use an array.

The purpose of an array is to store a set of values in an order. It does this using numeric property names. It has lots of special features specifically for handling numeric property names.

Use a regular object instead. You can convert it to an array you are iterate over with Object.entries, and filter out selected properties based on their name if you like.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const object = {};
object['test1'] = 'property 1';
object['test2'] = 'property 1';
object[0] = 'value 1';
object[1] = 'value 2';

const logger = ([name, value]) => console.log(`${name} is ${value}`);

Object.entries(object).forEach(logger);

console.log("-------");

Object.entries(object).filter(([name]) => !/^\d+$/.test(name)).forEach(logger);

<!-- end snippet -->

You can use it

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

var array = [];
array['test1'] = 'property 1';
array['test2'] = 'property 1';
array[0] = 'value 1';
array[1] = 'value 2';

Object.keys(array).forEach((key)=>{
  if(isNaN(+key)){
    console.log(array[key])
  }
})

<!-- end snippet -->

You can use a couple of conditions to filter out those you don't need:

function* nonNumericProperties(object) {
  for (const key in object) {
    // Own keys only.
    if (!Object.hasOwn(object, key)) {
      continue;
    }
    
    // Filter out non-negative integers.
    // Keys like '001' are valid.
    if (/^(?:[1-9]\d*|0)$/.test(key)) {
      
      // An array's size is capped at 2 ** 32 - 2,
      // which means integers greater than that
      // are not indices.
      // https://stackoverflow.com/a/6155063
      const parsedKey = Number(key);
      if (parsedKey <= 2 ** 32 - 2) {
        continue;
      }
    }
    
    yield [key, object[key]];
  }
}

Try it:

<!-- begin snippet: js hide: true -->

<!-- language: lang-js -->

console.config({ maximize: true });

function* nonNumericProperties(object) {
  for (const key in object) {
    if (
      !Object.hasOwn(object, key) ||
      (/^(?:[1-9]\d*|0)$/.test(key) && +key <= 2 ** 32 - 2)
    ) {
      continue;
    }
    
    yield [key, object[key]];
  }
}

const array = [0, 1, 2];
array.property = 'property';
array[Infinity] = Infinity;
array[1.5] = 'float';
array[2e-14] = 'float + scientific notation';
array['00234'] = '0-padded';
array[NaN] = NaN;
array[2 ** 32] = 2 ** 32;

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

<!-- language: lang-html -->

<script src="https://gh-canon.github.io/stack-snippet-console/console.min.js"></script>

<!-- end snippet -->