Handwriting can deepen the understanding of map, sometimes understand but not necessarily write, I hope we can do more, don’t play so much talk! Be a giant of action!

Map can pass in a callback function that returns a value. The map using

var arr = [1.2.3.4]
        var array = arr.map((item, index) = > {
            return item * 2
        })
        console.log(array);
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handwritten

var arr = [1.2.3.4];
function map(arr, mapCallback) {
            // 1: Check whether the parameters are correct
if (!Array.isArray(arr) || ! arr.length ||typeofmapCallback ! = ='function') {
                return[]}else {
                // Do the following
let result = []
for (let i = 0, len = arr.length; i <= len; i++) {
         result.push(mapCallback(arr[i], i, arr))
                }
                return result
            }
        }

        var sb = map(arr, (item) = > {
            return item * 2
        })
        console.log(sb);
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