background:
Multidimensional array expansion
question:
var arr = [1,2,[3,4,[5,6,[7,8],9],10]];
function fn(arr){
return [].concat(arr.map(d=> Array.isArray(d)?fn(d):d))//(3)[1, 2, Array(4)]
// return [].concat(...arr.map(a=> Array.isArray(a)? fn(a):a))//(10)[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
} . followed by the array, that is, the result of arr.map
is the difference between the two. The use of.
the first one. Arr [0], arr [1] will not say, arr [2] is that the array will execute fn, again no matter what the result is, the return statement returns an array, so arr [2] must be an array in the result, and then recursion is the same.
second, the return result executed by arr [2] is an array, but because there is a function parameter expander in front of it, the expansion of the result array executed by arr [2] will be passed to concat for connection. The following recursive principle is
. extension operation 12 [3,4, Array (4), 10]
map
teach you a simple way to execute the break point step by step
[].concat([1,[3]].map(d=> Array.isArray(d)? [].concat([3].map(a=>a)):d)) //[3].map(d=>d) [].concat([3]) == [3] //[1,[3]].map(d=> d) [].concat([1,[3]].map(d=> d)) [].concat([1,[3]]) [1,[3]]
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