JavaScript (ES6): Using spread operator inside conditional expression (?-operator)

Question

In JavaScript it's possible to merge array with another using the ... (spread) operator as follows

[...[1,2,3], ...[4,5,6]]
// -> [1, 2, 3, 4, 5, 6]

However, it seems ineligible to use the spread operator inside ternary ? expression

[true ? ...[1,2,3] : ...[4,5,6]]
// Uncaught SyntaxError: Unexpected token ...

Is this a justified restriction in the language, a deficiency or am I missing something? Is is possible to achieve this syntactically without imperative approach or Object.assign() function?

_"**Spread syntax** allows an iterable such as an array expression or string to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected, or an object expression to be expanded in places where zero or more key-value pairs (for object literals) are expected."_ ([Source](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax)) — Andreas, Jan 21 '19 at 16:34

score 11 · Accepted Answer · answered Jan 21 '19 at 16:34

Move the dots outside of the expression and perform the spreading with the result.

console.log([...(true ? [1, 2, 3] : [4, 5, 6])]);

This works only with values who are spreadable. If not, wrap the not spreadable part into an array.

console.log([...(true ? [1] : [4, 5, 6])]);

JavaScript (ES6): Using spread operator inside conditional expression (?-operator)

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