arr.sort();
console.log('欢迎'+arr.slice(0,arr.length-1).join(',')+"和"+arr[arr.length-1]+'同学');
console.log(`欢迎${arr.sort().slice(0,arr.length)}和${arr[arr.length-1]}同学！`);
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保持热爱
arr.sort();
console.log('欢迎'+arr.slice(0,arr.length-1).join(',')+"和"+arr[arr.length-1]+'同学');