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let flatArray = [].concat.apply([], arr);
//Output: [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 ]
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let flatArray = [].concat(arr);
//Output: [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 ]
//antohre way
let flatArray = arr.reduce((acc, curVal) => {
return acc.concat(curVal)
}, []);