Why can't I use javascript array rest destructuring both ways?

Question

I've been using and loving babel (6.5.2) for a while now and find the new destructuring syntax great for writing clearer JavaScript.

Why doesn't the rest destructuring work (it generates a token error) anywhere in the array? For example:

          const [column, ...restOfColumns] = columns;
          const objProps = column.valueChain.slice(0, -1);
          const prop = column.valueChain[column.valueChain.length - 1];
          //const [...objProps, prop] = column.valueChain

The commented out line would replace the preceding two lines with something much easier to read and understand.

The word rest should be the hint: it can only be used for the rest of the array. — Barmar
– Barmar, Commented Mar 27, 2017 at 20:09
In general, when arbitrary-length data is used, it's at the end of something, so they didn't design a full pattern matcher. — Barmar
– Barmar, Commented Mar 27, 2017 at 20:10
Thanks @Timo, I've raised this on [email protected] to see if I can get the spec to say different in future. — Paul Whipp
– Paul Whipp, Commented Mar 29, 2017 at 0:31

ChrisG · Accepted Answer · 2017-03-27 20:10:20Z

4

The simple answer is that when you use the destructing syntax ..., it means everything else. Therefore when you try [...objProps, prop], it doesn't know what to assign to prop as you have assigned all values already to objProps

answered Mar 27, 2017 at 20:10

ChrisG

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2 Comments

Paul Whipp Over a year ago

The ... syntax already works the way I suggest for objects. It is very handy being able to e.g. {x: 'x_default', ...original_object, ...object_revisions} for example. The ability to similarly construct and deconstruct lists is not just cool (although it is very cool), it is also logical and natural [first, ...rest, last] reads clearly and naturally for me.

Paul Whipp Over a year ago

This is a good answer, although I think the behaviour should be as posted in the question - changing it will mean a change to the specification in future.

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