微信小程序 数组中的push与concat的区别

var arr = []; 
arr.push(1); 
arr.push(2); 
arr.push([3, 4]) 
arr.push(5, 6); 
arr = arr.concat(7); 
arr = arr.concat([8, 9]); 
arr = arr.concat(10, 11); 
for(var i in arr){ 
 console.log(i+"-----"+arr[i]); 
} 

index.js [sm]:180 0-----1
index.js [sm]:180 1-----2
index.js [sm]:180 2-----3,4
index.js [sm]:180 3-----5
index.js [sm]:180 4-----6
index.js [sm]:180 5-----7
index.js [sm]:180 6-----8
index.js [sm]:180 7-----9
index.js [sm]:180 8-----10
index.js [sm]:180 9-----11