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Deep flatten a JavaScript array using Array.prototype.reduce
function flatten(array) {
return array.reduce( (acc, e) => {
if(Array.isArray(e)) {
// if the element is an array, fall flatten on it again and then take the returned value and concat it.
return acc.concat(flatten(e));
} else {
// otherwise just concat the value.
return acc.concat(e);
}, [] ) // initial value for the accumulator is []
flatten([1,2,3, [4,5, [6,7,[9,8]]]])
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