二维数组降维成一维数组

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let arr=[[2,5],[7,3]];
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arr.toString().split  //[2,5,7,3]

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function fn(arr){
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    let a=[];
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    arr.map(item=>{
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        a=[...a,...item]
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    });
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    return a
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}
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fn([[2,5],[7,3]])  //[2,5,7,3]

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let arr=[[1,2,3],[4,5,6],[7,8,9]].reduce(function(prev,next){
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    return prev.concat(next)
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})
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console.log(arr)//[1,2,3,4,5,6,7,8,9]