Cannot use spread inside conditional operator [duplicate]

Question

Suppose the following:

const arr = [1, 2, 3];
const insertValues = [-1, 0];
const condition = true;

arr.splice(0, 0, condition ? ...insertValues : insertValues);

This throws a syntax error:

Unexpected token '...'

I can get this to work by doing the following:

const arr = [1, 2, 3];
arr.splice(0, 0, ...[-1, 0]);

But this is obviously not what I'd like to do. How can I get the first example to work? I've tried including parentheses where I'd have thought necessary, to no avail.

Answer 1

It would work if ... were an operator, but it isn't (and can't be), it's primary syntax. Operators can't do what ... does. Operators have to have a single value result, but ... doesn't.

So instead, you need to spread an iterable. You could do that by wrapping insertValues in [] when you want to insert it directly (your falsy operand to the conditional operator):

arr.splice(0, 0, ...(condition ? insertValues : [insertValues]));
// (Or you could use `unshift` if the index will always be 0 and you're never deleting anything)

(That may be what you meant later in your question. It's not an unusual idiom in cases where you want to use something as a series of arguments that may or may not be an iterable.)

Or just use an if/else.

Answer 2

You can place ... before a conditional operator without parentheses:

const arr = [1, 2, 3];
const insertValues = [-1, 0];
const condition = true;
arr.splice(0, 0, ... condition ? insertValues : [insertValues]);

console.log(arr);

Since ... is a syntax (a syntax operates on results of expressions), operators are evaluated before it, a more explicit example:

console.log([... 'a' + 'b']);

Answer 3

Spread syntax is allowed in a handful of locations. Change your code to:

const arr = [1, 2, 3];
const insertValues = [-1, 0];
const condition = true;

arr.splice(0, 0, ...(condition ? insertValues : [insertValues]));

Or this:

if (condition)
  arr.splice(0, 0, ...insertValues);
else
  arr.splice(0, 0, insertValues);

Answer 4

If you don't want to bother with conditions and just want it work for both a number and an array, here's a solution that might be useful:

const arr = [1, 2, 3];
const insertValues = [-1, 0];
const condition = true;
arr.splice(0, 0, ...[insertValues].flat());

This is based on assumption that the intention was condition ? ...number[] : number