Is there a way to check Javascript function returning object / array is empty or not?

Question

What I am trying to achieve here is that, I made a simple utility function and that should return 'true' or 'false', based on the given argument to the isEmpty method.

// The below log should return 'false', But it's returning 'true'

console.log(isEmpty( () => {key: 1} ));

What I tried so far

 function isEmpty(value) {
  const type = typeof value;
    if ((value !== null && type === 'object') || type === 'function') {
       const properties = Object.keys(value);
        return properties.length === 0 || properties.size === 0
      } 
      return !value;
   }

And it's working for below cases

        console.log(isEmpty( {} )) // true
        console.log(isEmpty( [] )) // true
        console.log(isEmpty( Object.create(null) )) // true
        console.log(isEmpty( null )) // true
        console.log(isEmpty( '' )) // true

        console.log(isEmpty( {key: 1} )) // false
        console.log(isEmpty( [1,2,3] )) // false

But it's not working, when we get the return object / array from the function

    console.log(isEmpty( () => ({key: 1}) )) 
    console.log(isEmpty( () => ([1,2,3]) ))

If it is function you could execute the function and call isEmpty on the value returned. Something like if(type === 'function') return isEmpty(value()) ? — adiga
– adiga, Commented Nov 20, 2019 at 6:24
console.log(isEmpty( () => {key: 1} )) in this case you are passing a function not the return value. If you still want to check if the return value from function you have to invoke the function and check the same — Vineesh
– Vineesh, Commented Nov 20, 2019 at 6:24

Nick Parsons · Accepted Answer · 2019-11-20 07:28:10Z

2

Using Object.keys() on a function will always result in an empty array (no matter the return type). This is due to the fact that the keys of a function are not enumerable (the keys being name and length), and Object.keys() will only give back the keys which can be enumerated. This means the length of the array of keys will always be 0, meaning your function will give back true even though the function it is passed returns a non-empty value.

If you can invoke the value (if it is a function), it will allow you to get the returned value out of it (ie: an object or array), and then recurse with your function that you currently have:

function isEmpty(value) {
  const type = typeof value;
  if (value !== null && type === 'object') {
    const prototype = Object.getPrototypeOf(value) || {};
    const properties = Object.keys(value) + Object.keys(prototype);
    return properties.length === 0 || properties.size === 0
  } else if(type === 'function') {
    const res = value();
    return isEmpty(res);
  }
  return !value;
}

console.log(isEmpty( {} )) // true
console.log(isEmpty( [] )) // true
console.log(isEmpty( Object.create(null) )) // true
console.log(isEmpty( null )) // true
console.log(isEmpty( '' )) // true

console.log(isEmpty(() => ({}))); // true
console.log(isEmpty(() => () => () => ({}))); // true

console.log(isEmpty( {key: 1} )) // false
console.log(isEmpty( [1,2,3] )) // false

console.log(isEmpty(() => ({key: 1}))); // false
console.log(isEmpty( () => ([1,2,3]) )) // false


console.log(isEmpty(() => (Object.create({key: 1})))) // false

edited Nov 20, 2019 at 7:28

answered Nov 20, 2019 at 6:27

Nick Parsons

51.7k6 gold badges61 silver badges80 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

RajuPedda Over a year ago

And it's not working for this scenario - console.log(isEmpty( () => (Object.create({key: 1}) )));

Nick Parsons Over a year ago

@RajuPedda see edit, (you need to account for the enumerable properties on the prototype as well)

TryingToImprove · Accepted Answer · 2019-11-20 06:59:21Z

1

In order to achieve that you will have to check the type of the value and if it is a function then invoke to get the result.

If the function passed have side effects they would be called, which could cause some issues.

function isEmpty(value) {
    const type = typeof value;

    // If passed a function, invoke it and get the result
    if (type === 'function') {
        value = value();
    }

    if (value && (type === 'object' || type === 'function')) {
        const properties = Object.keys(value);
        return properties.length === 0 || properties.size === 0
    } 

    return !value;
}

When you call a function like () => { key: 1 } you are actually creating a function which looks like

function {
    key: 1
}

which means that the function do not have a return value. Instead you should use it like this () => ({ key: 1 }) which will create a function like:

function {
    return { key: 1 }
}

https://babeljs.io/repl#?babili=false&browsers=&build=&builtIns=false&spec=false&loose=false&code_lz=MYewdgzgLgBAYgVzMAjDAvDAFASgwPhgG8YBrAUwE8AuGNAXwChRJZFkAmDbPdQrEhRp0Y9HEA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=true&presets=es2015%2Creact%2Cstage-2&prettier=false&targets=&version=7.7.3&externalPlugins=

edited Nov 20, 2019 at 6:59

answered Nov 20, 2019 at 6:28

TryingToImprove

7,4555 gold badges32 silver badges40 bronze badges

3 Comments

RajuPedda Over a year ago

It's throwing error, when I send null value console.log(isEmpty( () => (null )));

TryingToImprove Over a year ago

Sorry, had !value instead of value.. It should work now :)

RajuPedda Over a year ago

And it's not working for this scenario - console.log(isEmpty( () => (Object.create({key: 1}) )));

Collectives™ on Stack Overflow

Is there a way to check Javascript function returning object / array is empty or not?

2 Answers 2

2 Comments

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related