Setting value of key in array based on value of another array

Question

I've attached a codepen (http://codepen.io/anon/pen/PGkQmq) of a working concept of what I'm trying to accomplish. This working code assigns a name to an null value of the name key in the JSON object based on the respective position in the defaultNameArray.

My problem is with code in which I attempt to generate the defaultObj array in code for a length equal to the defaultNameArray.

var defaultNameArray = ["Bob", "Alice", "Joe"];

var defaultObj = [
  {
    "name": null,
    "age": "32",
    "height": "175",
    "etc": "someData",
  },
];

for (var i = 0; i < defaultNameArray.length -1; i++) {
    defaultObj.push(defaultObj[0]);
}

for (var i = 0; i < defaultObj.length; i++) {
    defaultObj[i].name = defaultNameArray[i];
    console.log("defaultObj[i].name: " + defaultObj[i].name);
    console.log("defaultNameArray[i]" + defaultNameArray[i]);
};

var str = JSON.stringify(defaultObj);
console.log('defaultObj: ' + str);

This code generates a generic array that I should then be able to iterate through to assign values to a key, with respect to the position in another array (names). The end result is all of the names being "Joe", the last position in the names array. Why does iterating through a generated array of JSON behave differently than using one defined in the code like the attached codepen, and what am I missing?

Don't call a plain old JavaScript object "JSON". Call it a POJO if you really need an acronym for it. The string you create at the end is JSON. — trincot
– trincot, Commented Sep 13, 2016 at 14:52

trincot · Accepted Answer · 2016-09-13 14:55:16Z

2

You push the same object defaultObj[0] to the array again and again. If you mutate that object, that change will be seen, no matter from which array index you look at it. It is the same, single object. If you change defaultObj[0].name, you change defaultObj[1].name: it's the same thing.

So, you need to copy (clone) your object. For that you can use Object.assign, which performs a shallow copy -- enough for your case:

Change:

defaultObj.push(defaultObj[0]);

to:

defaultObj.push(Object.assign({}, defaultObj[0]));

var defaultNameArray = ["Bob", "Alice", "Joe"];

var defaultObj = [
  {
    "name": null,
    "age": "32",
    "height": "175",
    "etc": "someData",
  },
];

for (var i = 0; i < defaultNameArray.length -1; i++) {
    defaultObj.push(Object.assign({}, defaultObj[0]));
}

for (var i = 0; i < defaultObj.length; i++) {
    defaultObj[i].name = defaultNameArray[i];
    console.log("defaultObj[i].name: " + defaultObj[i].name);
    console.log("defaultNameArray[i]" + defaultNameArray[i]);
};

var str = JSON.stringify(defaultObj);
console.log('defaultObj: ' + str);

edited Sep 13, 2016 at 14:55

answered Sep 13, 2016 at 14:49

trincot

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2 Comments

Bryan Over a year ago

@trincot Thank you for your explanation! +1

trincot Over a year ago

You're welcome. I was a bit surprised you did not accept it and went for an answer that was posted 8 minutes later, but the vote compensates :)

taguenizy · Accepted Answer · 2016-09-13 14:57:51Z

1

defaultObj.push(defaultObj[0]);
This pushes the same object with it's reference to the array so when you change one property in one of them you change in all. That's why in the end they only have the value of the last element.

Try defaultObj.push(Object.assign({}, defaultObj[0]));

Object assign documentation

answered Sep 13, 2016 at 14:57

taguenizy

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