如何将所有数组合并为一个数组？

var arr1 = [1, 2, 3];
var arr2 = [4, 5, 6];
var arr3 = [7, 8, 9];

var mergedArray = arr1.concat(arr2, arr3);
console.log(mergedArray);

var arr1 = [1, 2, 3];
var arr2 = [4, 5, 6];
var arr3 = [7, 8, 9];

var mergedArray = [...arr1, ...arr2, ...arr3];
console.log(mergedArray);

var arrays = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];

var mergedArray = arrays.reduce(function(result, currentArray) {
  return result.concat(currentArray);
}, []);
console.log(mergedArray);