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I want to create an array of objects and then build a JSON string. I have the following code. After trying many times, I found the solution. Can anybody explain the difference between create and the new operator? It looks to me create is making a copy of the object.

HmiUserList = [];
var oco = {};
//If I use the following line it gives wrong string
//oco = Object.create({});
for (var i = 0; i < 3; i++) {
//If I use the following gives right string
oco = new Object();
oco.UserName = i.toString();
oco.GroupName = (i + 2).toString();
var count = HmiUserList.push(oco);
console.log(JSON.stringify(HmiUserList));
}

//Wrong string if I use the create or if I don't use the new for oco
[{"UserName":"2","GroupName":"4"},{"UserName":"2","GroupName":"4"},{"UserName":"2","GroupName":"4"}]

//Target string [{"UserName":"0","GroupName":"2"},{"UserName":"1","GroupName":"3"},{"UserName":"2","GroupName":"4"}]
RotatingWheel
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  • Likely a dup of https://stackoverflow.com/questions/2709612/using-object-create-instead-of-new – danh Oct 11 '21 at 19:51

1 Answers1

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The difference in behaviour is here not related to using Object.create or new. It is caused by where you put the assignment to oco: in the loop or before the loop.

When you do it before the loop, there is only one object, which the loop keeps mutating, iteration after iteration, and it pushes that same object in the output array. Even though the array ends up with 3 object references, all three represent the same object.

When you do it inside the loop, you create a new object in each iteration, so that your property assignments affect that object, and not an object that was already pushed onto the array. Your array ends up with 3 different objects as intended.

trincot
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