ES6 spread element - default value

Question

I need to set an empty object as a default value if the array I'm passing in is empty. Something like:

var obj = { documents: [...question.documents] || [{}] }

I fixed it using a condition, but I want to know if there is a better way to achieve that.

if(obj.documents.length === 0) obj.documents.push({})

There is no such thing as a speed operator: https://stackoverflow.com/questions/37151966/what-is-spreadelement-in-ecmascript-documentation-is-it-the-same-as-spread-oper/37152508#37152508 — Felix Kling, Apr 14 '18 at 14:59

CertainPerformance · Answer 1 · 2018-04-14T01:00:39.343

Since even empty arrays are truthy, I don't think there's any great elegant solution other than putting an explicit test in there somewhere. Ternaries are more terse than if statements, though:

const question = { documents: [] };
const { documents } = question;
const obj = { documents: documents.length !== 0 ? documents : [{}]}
console.log(JSON.stringify(obj));

Here's another possibility:

const question = { documents: [] };
const [firstElm = {}, ...otherElms] = question.documents;
const obj = { documents: [firstElm, ...otherElms] };
console.log(obj);

Aluan Haddad · Answer 2 · 2018-04-14T02:03:51.490

1

There are a couple of ways to write this in a single expression

Using the ternary operator:

var obj = { documents: [
  ...question.documents.length
    ? question.documents
    : [{}]
  ] 
};

Using a default value

var obj = { documents: [question.documents[0] || {}, ...question.documents.slice(1)] };

In both cases there's some awkwardness stemming from having to refer to the source multiple times

edited Apr 14 '18 at 02:03

answered Apr 14 '18 at 00:56

Aluan Haddad

26,341
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The second duplicates the first item in the array while spreading: https://jsfiddle.net/ks17peb2/ – CertainPerformance Apr 14 '18 at 00:57
@CertainPerformance great catch thank you. I totally forgot by the time I was halfway through. Fixed! – Aluan Haddad Apr 14 '18 at 00:58

Akash · Answer 3 · 2018-04-14T09:01:47.743

The spread operator is used inside an empty array. I don't see the point in using the spread operator here. The objective can be achieved by using the following.

var obj = { documents: question.documents.length ? question.documents : [{}]}

If the method you have provided is being used, you don't need an or clause, because an empty array also returns a truthy value. So it can be written as the following :-

var obj = { documents: question.documents }
if(!obj.documents.length) obj.documents.push({})

score 0 · Answer 4 · answered Apr 14 '18 at 09:05

0

this should suit...

const question = {
  documents: [],
};

const obj = { 
  documents: [].concat(question.documents.length ? question.documents : {}) 
};

console.log(obj);

answered Apr 14 '18 at 09:05

Hitmands

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3
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ES6 spread element - default value

4 Answers4