Develop Reference

Trying to select property from array within object, within array. Thrown TypeError: Cannot read properties of undefined

I am trying to make a project with React that can display course homework on a to-do list. I currently have some test data, shown here:
const courses = [
{
courseName: "Fundies 2",
classCode: "CS2510",
startTime: "4:35 PM",
location: "236 Richards Hall",
color: "red",
homework: [{ hwName: "HW3", timeDue: "8:30 PM" }]
},
{
courseName: "Mathematical Reasoning",
classCode: "MATH1365",
startTime: "10:20 AM",
location: "235 Ryder Hall",
color: "blue",
homework: [
{ hwName: "HW2", timeDue: "midnight" },
{ hwName: "HW3", timeDue: "midnight" },
]
},
];
I made a function to create a component out of this data, shown below.
const formattedCourses = courses.map((course) => {
return (
<Task
key={course.homework[1].hwName}
timeDue={course.homework[1].timeDue}
courseName={course.courseName}
hwName={course.homework[1].hwName}
color={course.color}
/>
);
});
I want the component to be made with the first entry in the homework array. However, whenever I try to compile, I get
"TypeError: Cannot read properties of undefined (reading 'hwName')"
How can I fix this?
Any and all help is greatly appreciated!

You index is incorrect. If you want the first entry then it should be 0 instead of 1
const formattedCourses = courses.map((course) => {
return (
<Task
key={course.homework[0].hwName}
timeDue={course.homework[0].timeDue}
courseName={course.courseName}
hwName={course.homework[0].hwName}
color={course.color}
/>
);
});

Index of nested array

I had an array like this
var data = [
{
name:"Movies",
info: "category_name",
data: [
{
name:"Interstellar",
info: "category_data"
},
{
name:"Dark Knight",
info: "category_data"
},
]
},
{
name:"Music",
info: "category_name",
data: [
{
name:" Adams",
info: "category_data"
},
{
name:"Nirvana",
info: "category_data"
},
]
}
]
console.log(data[0]);
If I console.log ([1][0]) the answer is undefined. I want to show "Interstellar" or "category data". Which is the index of this array?

data[1][0] is undefined because data[1] is will give you the second object in the array, data[1][0] wont give you the first value in data[1], instead, data[1].name will be the first value in your object. The square brackets with index are only applicable to arrays

How to get all values of given specific keys (for e.g: name) without loop from json?

I want to fetch all the names and label from JSON without loop. Is there a way to fetch with any filter method?
"sections": [
{
"id": "62ee1779",
"name": "Drinks",
"items": [
{
"id": "1902b625",
"name": "Cold Brew",
"optionSets": [
{
"id": "45f2a845-c83b-49c2-90ae-a227dfb7c513",
"label": "Choose a size",
},
{
"id": "af171c34-4ca8-4374-82bf-a418396e375c",
"label": "Additional Toppings",
},
],
},
]
}

When you say "without loops" I take it as without For Loops. because any kind of traversal of arrays, let alone nested traversal, involve iterating.
You can use the reduce method to have it done for you internally and give you the format you need.
Try this :
const data = {
sections: [
{
id: "62ee1779",
name: "Drinks",
items: [
{
id: "1902b625",
name: "Cold Brew",
optionSets: [
{
id: "45f2a845-c83b-49c2-90ae-a227dfb7c513",
label: "Choose a size"
},
{
id: "af171c34-4ca8-4374-82bf-a418396e375c",
label: "Additional Toppings"
}
]
}
]
}
]
};
x = data.sections.reduce((acc, ele) => {
acc.push(ele.name);
otherName = ele.items.reduce((acc2, elem2) => {
acc2.push(elem2.name);
label = elem2.optionSets.reduce((acc3, elem3) => {
acc3.push(elem3.label);
return acc3;
}, []);
return acc2.concat(label);
}, []);
return acc.concat(otherName);
}, []);
console.log(x);
Go ahead and press run snippet to see if this matches your desired output.
For More on info reduce method

In the context of cJSON
yes, we can fetch the key value for any of the object.
1 - each key value is pointed by one of the objects. will simply fetch that object and from there will get the key value.
In the above case for
pre-requisition: root must contain the json format and root must be the cJSON pointer. if not we can define it and use cJSON_Parse() to parse the json.
1st name object is "sections" will use
cJSON *test = cJSON_GetObjectItem(root, "sections");
char *name1 = cJSON_GetObjectItem(test, "name" )->valuestring;
2nd name key value
cJSON *test2 = cJSON_GetObjectItem(test, "items");
char *name2 = cJSON_GetObjectItem(tes2, "name")->valuestring;
likewise, we can do for others as well to fetch the key value.

getting a specific value from JSON array with multiple arrays inside in javascript

I'm asking here as I can see this website the most one can help in this
I have an output value in JASON format as the following:
{
"total": 16,
"members": [{
"id": 4,
"name": "Blade11",
"descriptors": {
"os": "Windows 2012 / WS2012 R2"
},
"FCPaths": [{
"wwn": "50060B0000C27208",
"hostSpeed": 0
}, {
"wwn": "50060B0000C2720A",
"hostSpeed": 0
}],
"iSCSIPaths": [],
"persona": 11,
"links": [{
"href": "https://3par:8080/api/v1/hostpersonas?query=\"wsapiAssignedId EQ 11\"",
"rel": "personaInfo"
}],
"initiatorChapEnabled": false,
"targetChapEnabled": false
}, {
"id": 6,
"name": "Blade4",
"descriptors": {
"os": "VMware (ESXi)"
},
"FCPaths": [{
"wwn": "50060B0000C27216",
"hostSpeed": 0
}, {
"wwn": "50060B0000C27214",
"hostSpeed": 0
}],
"iSCSIPaths": [],
"persona": 8,
"links": [{
"href": "https://3par:8080/api/v1/hostpersonas?query=\"wsapiAssignedId EQ 8\"",
"rel": "personaInfo"
}],
"initiatorChapEnabled": false,
"targetChapEnabled": false
}
what I want is, to parse this output for retrieving an output parameter with the name object only in a list or array of string
for example Names = Blade11, Blade4,....
if you can see in the Json output we have all the names under "members", then each one is another array of values, I want to retrieve only names
thanks

If this JSON is string first you have to parse it
var json = JSON.parse('here is your JSON string');
Than you can access to it properties like you work with object
var names = json.members.map(function(member) {
return member.name;
});

Since you already have JSON format, you can use an array method on the member key of your JSON object to iterate through each array item.
var names = [];
object_name.members.forEach((arrItem) => {
names.push(arrItem.name);
}
or
namesArray = object_name.members.map((arrItem) => {
return arrItem.name;
}

You could use Array#map for iterating all elements of the array and return only the name property.
If you have a JSON string, you need to parse it in advance for getting an object, like
object = JSON.parse(jsonString);
var jsonString = '{"total":16,"members":[{"id":4,"name":"Blade11","descriptors":{"os":"Windows 2012 / WS2012 R2"},"FCPaths":[{"wwn":"50060B0000C27208","hostSpeed":0},{"wwn":"50060B0000C2720A","hostSpeed":0}],"iSCSIPaths":[],"persona":11,"links":[{"href":"https://3par:8080/api/v1/hostpersonas?query=\\"wsapiAssignedId EQ 11\\"","rel":"personaInfo"}],"initiatorChapEnabled":false,"targetChapEnabled":false},{"id":6,"name":"Blade4","descriptors":{"os":"VMware (ESXi)"},"FCPaths":[{"wwn":"50060B0000C27216","hostSpeed":0},{"wwn":"50060B0000C27214","hostSpeed":0}],"iSCSIPaths":[],"persona":8,"links":[{"href":"https://3par:8080/api/v1/hostpersonas?query=\\"wsapiAssignedId EQ 8\\"","rel":"personaInfo"}],"initiatorChapEnabled":false,"targetChapEnabled":false}]}',
object = JSON.parse(jsonString),
array = object.members.map(function (a) { return a.name; });
console.log(array);

MongoDB : Map Reduce : Create one sub-document from another one

I have a mongodb collection which has documents like this :
{
"_id" : ObjectId("safdsd435tdg54trgds"),
"startDate" : ISODate("2013-07-02T17:35:01.000Z"),
"endDate" : ISODate("2013-08-02T17:35:01.000Z"),
"active" : true,
"channels" : [
1, 2, 3, 4
],
}
I want to convert this to something like this :
{
"_id" : ObjectId("safdsd435tdg54trgds"),
"startDate" : ISODate("2013-07-02T17:35:01.000Z"),
"endDate" : ISODate("2013-08-02T17:35:01.000Z"),
"active" : true,
"channels" : [
1, 2, 3, 4
],
"tags" :[
{
"name": one
"type": channel
},
{
"name": two
"type": channel
},
{
"name": three
"type": channel
},
{
"name": four
"type": channel
}
]
}
I already have a mapping of what 1,2,3,4 mean. Just for the sake of simplicity I put them as their alphabetical format. the values could be different, but they're static mappings.

You seem to be trying to do this update without a big iteration of your collection, So you "could" do this with mapReduce, albeit in a very "mapReduce way" as it has it's own way of doing things.
So first you want to define a mapper that encapsulates your current document :
var mapFunction = function (){
var key = this._id;
var value = {
startDate: this.startDate,
endDate: this.endDate,
active: this.active,
channels: this.channels
};
emit( key, value );
};
Now here the reducer is actually not going to be called as all the keys from the mapper will be unique, being of course the _id values from the original document. But to make the call happy:
var reduceFunction = function(){};
As this is a one to one thing this will go to finalize. It could be in the mapper, but for cleanliness sake
var finalizeFunction = function (key, reducedValue) {
var tags = [
{ name: "one", type: "channel" },
{ name: "two", type: "channel" },
{ name: "three", type: "channel" },
{ name: "four", type: "channel" }
];
reducedValue.tags = [];
reducedValue.channels.forEach(function(channel) {
reducedValue.tags.push( tags[ channel -1 ] );
});
return reducedValue;
};
Then call the mapReduce:
db.docs.mapReduce(
mapFunction,
reduceFunction,
{
out: { replace: "newdocs" },
finalize: finalizeFunction
}
)
So that will output to a new collection, but in the way that mapReduce does it so you have this:
{
"_id" : ObjectId("53112b2d0ceb66905ae41259"),
"value" : {
"startDate" : ISODate("2013-07-02T17:35:01Z"),
"endDate" : ISODate("2013-08-02T17:35:01Z"),
"active" : true,
"channels" : [ 1, 2, 3, 4 ],
"tags" : [
{
"name" : "one",
"type" : "channel"
},
{
"name" : "two",
"type" : "channel"
},
{
"name" : "three",
"type" : "channel"
},
{
"name" : "four",
"type" : "channel"
}
]
}
}
So all your document fields other than _id are stuck under that value field, so that's not the document that you want. But that is how mapReduce works.
If you really need to get out of jail from this and are willing to wait a bit, the upcoming 2.6 release has added an $out pipeline stage. So you "could" transform the documents in your new collection with $project like this:
db.newdocs.aggregate([
// Transform the document
{"$project": {
"startDate": "$value.startDate",
"endDate": "$value.endDate",
"active": "$value.active",
"channels": "$value.channels",
"tags": "$value.tags"
}},
// Output to new collection
{"$out": "fixeddocs" }
])
So that will be right. But of course this is not your original collection. So to back to that state you are going to have to .drop() collections and use .renameCollection() :
db.newdocs.drop();
db.docs.drop();
db.fixeddocs.renameCollection("docs");
Now please READ the documentation carefully on this, there are several limitations, and of course you would have to re-create indexes as well.
All of this, and in particular the last stage is going to result in a lot of disk thrashing and also keep in mind that you are dropping collections here. It almost certainly is a case for taking access to your database off-line while this is performed.
And even as such the dangers here are real enough that perhaps you can just live with running an iterative loop to update the documents, using arbitrary JavaScript. And if you really must have to do so, you could always do that using db.eval() to have that all execute on the server. But if you do, then please read the documentation for that very carefully as well.
But for completeness even if I'm not advocating this:
db.eval(function(){
db.docs.find().forEach(function(document) {
var tags = [
{ name: "one", type: "channel" },
{ name: "two", type: "channel" },
{ name: "three", type: "channel" },
{ name: "four", type: "channel" }
];
document.tags = [];
document.channels.forEach(function(channel) {
document.tags.push( tags[ channel -1 ] );
});
var id = document._id;
delete document._id;
db.docs.update({ "_id": id },document);
});
})