数组去重+ 数组扁平化常用方法
数组去重：
var arr = [1,1,'true','true',true,true,15,15,false,false, undefined,undefined, null,null, NaN, NaN,'NaN', 0, 0, 'a','a',{a:1,b:2},{a:1,b:2}
];
//1
function unique(arr){
var obj = {}
return arr.filter(function(item, index, arr){
return obj.hasOwnProperty(typeof item + item) ?
false : (obj[type item + item] = true)
})
}
//2
arr1 = [... new Set(arr)]
//3
arr2 = Array.from(new Set(arr))
//4
let obj = {}
arr.forEach(item => {
obj[item] = 1
})
Object.keys(obj).map(o => Number(o)) 数组扁平化
//1
function flatten(arr){
return arr.toString().split(',').map(item => {
return Number(item)
})
}
//2
function flatten(arr){
return arr.join(',').split(',').map(item => {
return parseInt(item)
})
}
//3
arr1 = arr.flat(Infinity)
//4
function flatten(item){
return items.reduce((prev, current) => {
return prev.concat(Array.isArray(current) ?
flatten(current) : current
)
}, [])
} 
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