In Perl I can say:

@a = ();
unshift @a;                # @a is still ()
unshift @a, ();            # @a is still ()
unshift @a, 1>0 ?  1 : 0;  # @a is now (1)
unshift @a, 1>0 ? () : 0;  # @a is still (1)

What, if it exists, is the JavaScript equivalent syntax?

a=[];
a.unshift();             # a is still []
a.unshift(...[]);        # a is still []
a.unshift( 1>0 ? 1 : 0); # a is now [1]

a.unshift( 1>0 ?  ...[]  : 0 );  # syntax error
a.unshift( 1>0 ? (...[]) : 0 );  # syntax error
a.unshift( 1>0 ?     []  : 0 );  # a is now [[],1]

In Perl I can say:

@a = ();
unshift @a;                # @a is still ()
unshift @a, ();            # @a is still ()
unshift @a, 1>0 ?  1 : 0;  # @a is now (1)
unshift @a, 1>0 ? () : 0;  # @a is still (1)

What, if it exists, is the JavaScript equivalent syntax?

a=[];
a.unshift();             # a is still []
a.unshift(...[]);        # a is still []
a.unshift( 1>0 ? 1 : 0); # a is now [1]

a.unshift( 1>0 ?  ...[]  : 0 );  # syntax error
a.unshift( 1>0 ? (...[]) : 0 );  # syntax error
a.unshift( 1>0 ?     []  : 0 );  # a is now [[],1]

• javascript

• 1 You might check on the documentation for unshift(). – mykaf Commented Mar 3 at 14:10
• What is a.unshift( 1>0 ? (...[]) : 0 ); supposed to do? Do you understand what ... does? – luk2302 Commented Mar 3 at 14:12
• 1 @luk2302 I want the equivalent of the Perl lines I showed. In particular, the fourth line – jhnc Commented Mar 3 at 14:13
• 2 a.unshift( ...(1>0 ? [] : [0]) ); – luk2302 Commented Mar 3 at 14:16
• 1 unshift @a, 0 if 1 > 0; and if ( 1 > 0 ) a.unshift( 0 ); are a lot clearer, though. – ikegami Commented Mar 3 at 15:49

1 Answer 1

You cannot use the spread syntax where you have it, as an operand of the conditional (ternary) operator. Instead inverse the order of the spread syntax and the conditional operator and make sure the conditional operator always evaluates to an iterable -- so provide [0] instead of 0:

a.unshift(...(1 > 0 ? [] : [0]))

As Mozilla Contributors wrote:

the spread (...) syntax allows an iterable, such as an array or string, to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected.

The operand of the conditional operator is not such a place as exactly one operand is expected between the ? and : symbols.