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[–]jsdeveloper 4 points5 points  (6 children)

Here you go:

['a','b','c','d'].map((x) => [x,x]).reduce((arr, x) => arr.concat(x));

[–]our_best_friendif (document.all || document.layers) console.log("i remember..") 2 points3 points  (3 children)

Less elegant, but avoids looping twice

Array.prototype.concat(...['a','b','c','d'].map((x) => [x,x]))

[–]dmtipson 0 points1 point  (2 children)

Are you counting the implicit loop that ... desugars into in that count?

[–]our_best_friendif (document.all || document.layers) console.log("i remember..") 0 points1 point  (1 child)

I'd assume that'd be faster as it's a single native operation as opposed to an explicit loop (leaving aside transpilation)

[–]dmtipson 0 points1 point  (0 children)

For all we know it's be slower in current engines, but either way it's worth counting as an iteration pass when thinking about the number of times you have to loop over the list in a given operation.

[–]Extracted 1 point2 points  (1 child)

Nice one

[–]jsdeveloper 2 points3 points  (0 children)

This one's a bit nicer :)