For Loop vs. in loop

JSON is a data format in JS.
var obj={a:15,b:8,c:12} JSON Array object
var arr=[15,8,12]; Array
alert (OBJ.A); ---15
Alert (obj[' a ']);---15
Alert (arr[0]);---15

for (Var i=0;i<arr.length;i++) {//array loop first notation
Alert (i+ "=" +arr[i]);
} popup: 0=15 1=8 2=12

for (var i in arr) {//array loop second notation
}

for (var i in obj) {//json loop
Alert (i+ "=" +obj[i]);
} popup: A=15 b=8 c=12

Summary: Arrays can be used for loops or for in loops (the difference: The For loop can set the start number of I, can start a loop from any position, but the for in loop cannot be set, only from the first to the last).
JSON can only be used for the in loop because the JSON subscript is an irregular string with no length.
Therefore, the general array is used for loops, and JSON is used for the in loop.

For Loop vs. in loop