完成数组的拼接和截取操作

输出 (3) [1, 2, 3] 0: 1 1: 2 2: 3 (5) [1, 2, 3, “hello”, null] 0: 1 1: 2 2: 3 3: “hello” 4: null false 输出 (3) [1, 2, 3] 0: 1 1: 2 2: 3 length: 3 (9) [1, 2, 3, 4, 5, 6, 7, 8, 9] 0: 1 1: 2 2: 3 3: 4 4: 5

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