I am pulling an object from backbone.js and when I stringify the object I see string literal
'[{"Name":"Testname","Address":"Testaddress","id":"444444444444444"}]'
However, when I assign the non-serialized object to a variable and try to access the 0th element, I get undefined. I would expect to get object
{"Name":"Testname","Address":"Testaddress","id":"444444444444444"}
Is JavaScript not treating
[{"Name":"Testname","Address":"Testaddress","id":"444444444444444"}]
as an indexed array of objects?
To access elements of Backbone.Collection by index, use the Collection#at method:
var first = collection.at(0);
Alternatively, you can use the Collection#first method, which is actually part of the underscore library, but is proxied to Backbone collections for syntactic sugar:
var first = collection.first();
The reason you're seeing the array representation in the serialized JSON is that by convention JSON.stringify looks for a method called toJSON on the object you give to it to stringify, and if one is found, the return value of that method will be used instead. The implementation of Collection#toJSON returns a clone of the collection's internal array of models, and thus the JSON output is an array.
Just tried
var arr = JSON.parse( '[{"Name":"Testname","Address":"Testaddress","id":"444444444444444"}]' );
and
console.log( arr[0] ); // => object
What you've described should work.
Related
For example, if I have an object like:
{"angeredoutsidecontrol":"1","difficultiespileup":"2"}
And then later in a for loop I can access the key of angeredoutsidecontrol , how can I get the value returned as 0, which would represent which place in the object this key is?
There is no guaranteed order for keys of an object.
Definition of object from an old - but still effective in this case - documentation:
4.3.3 Object
An object is a member of the type Object. It is an unordered
collection of properties each of which contains a primitive value,
object, or function. A function stored in a property of an object is
called a method.
If order really matters to you, use array instead. For example:
[{ "angeredoutsidecontrol": "1" }
{ "difficultiespileup": "2" }];
The order of JSON objects is not maintained, so you can't do this.
[Is the order of elements in a JSON list maintained?
var myMoods = ["angeredoutsidecontrol","difficultiespileup"];
and
myMoods.indexOf( 'angeredoutsidecontrol' )
gives you position in your list
I'm learning js, find this code:
var arr = [
{id: 111, now: '12.02.2014'}
];
What is this? I know that var arr = [ ... ] - array, but what is {} in array and how i can work with this data and display this ?
{} is the syntax for creating an object. It's called an object initializer, but it's frequently called an "object literal".
So what you're doing there is creating an object which has id and now properties, and putting that object into an array as its only entry.
...how i can work with this data and display this ?
To display the id, for instance:
console.log(arr[0].id);
What that does:
arr[0] - retrieve the first entry in the array. In our case, it's an object.
.id - Get the value of the id property from that object.
We could also write it like this:
var obj = arr[0];
console.log(obj.id);
Alternately, if we didn't know in advance what property we wanted but we were given a string containing the name of the property, we could use [] with the object as well:
var nameOfProperty = "id";
var obj = arr[0];
console.log(obj[nameOfProperty]);
JavaScript has both the dotted syntax (obj.id), and the bracketed syntax (obj["id"]) for accessing object properties, where with the latter you can use any string (including one from a variable).
Yes, that is an object inside an array. In truth, all values, from numbers to functions to arrays, are actually objects.
You can access this object in the same way as you would any item of an array. (arr[0])
You can then access properties of the object, for example arr[0].id.
For more about objects, take a look at Objects on MDN.
I want to serialize in json a non associative array and the output is quite disturbing
JSON.stringify([1]);
// Expected : "[1]"
Output : "\"[1]\""
It treats the array as a string, what am i missing ?
I'm using Chrome Version 29.0.1547.65
The issue you are seeing is because of an Array.prototype.toJSON method that has been defined incorrectly with respect to the semantics of JSON.stringify. See below:
From: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify
toJSON behavior
If an object being stringified has a property named toJSON whose value
is a function, then the toJSON method customizes JSON stringification
behavior: instead of the object being serialized, the value returned
by the toJSON method when called will be serialized.
When an object has a toJSON method the result of that method will be stringified in its place. If the toJSON method is defined as a stringification then the object will be double stringified.
The only work-around I am aware of is to remove the method or to implement your own stringify() method with different semantics than the built-in.
If you can, simply remove the method from Array.prototype. If you are concerned this will break other functionality on the page then you need to remove it, stringify, then restore it.
function myStringify( o ) {
var temp = Array.prototype.toJSON;
delete Array.prototype.toJSON;
var result = JSON.stringify(o);
Array.prototype.toJSON = temp;
return result;
}
I am working on a function that accepts a JSON object as a parameter. Each property of the object passed in will be an array. I need to determine the length of each of these arrays.
What I won't know in advance are: the names of the properties, or how many properties there are.
How can I determine the length of each array? Again, I can't refer to any of the properties/keys explicitly because I won't know their names ahead of time. How do I reference each unknown property as an array?
Thank you in advance,
-RS
You can iterate over the object using a for...in loop to access each property.
for (var key in myObject) {
// Check to make sure the property is not part of the `Object` prototype (eg. toString)
if (myObject.hasOwnProperty(key)) {
console.log(key, myObject[key].length);
}
}
In the above example, we iterate over the properties of the object myObject and we log out their length values (assuming they are all arrays).
If by "JSON object" you mean that it is a string of JSON, you can first parse myObject into a true object by using myObject = JSON.parse(myObject) (which works in modern browsers) or myObject = eval("(" + myOjbect + ")") which is considered unsafe (but works in all browsers). After parsing, you may use the above technique to fine array lengths.
Note: Since you updated the post to specify that the object is JSON, you may not need the hasOwnProperty check, but it is safer to always use this test.
Use JSON.parse to convert JSON data to JS Objects, and then you can use Array's length etc properties.
var myObject = JSON.parse(myJSONtext, reviver);
JSON Reference: http://www.json.org/js.html
I have a js 'associative' array, with
array['serial_number'] = 'value'
serial_number and value are strings.
e.g. array['20910930923'] = '20101102'
I sorted it by value, works fine.
Let's say I get back the object 'sorted';
Now I want to access the first KEY of the 'sorted' array.
How do I do it? I can't think I need an iteration with
for (var i in sorted)
and just stop after ther first one...
thanks
edit: just to clarify, I know that js does not support associative arrays (that's why I put it in high commas in the Title).
2021 Update
Since ES6, properties with string keys are enumerated in insertion order. Here's a nice summary. My original answer from 2010 was correct at the time and is preserved below:
Original answer
JavaScript object properties are specified to have no order, much though many people wish it were different. If you need ordering, abandon any attempt to use an object and use an Array instead, either to store name-value objects:
var nameValues = [
{name: '20910930923', value: '20101102'},
{name: 'foo', value: 'bar'}
];
... or as an ordered list of property names to use with your existing object:
var obj = {
'20910930923': '20101102',
'foo': 'bar'
};
var orderedPropertyNames = ['20910930923', 'foo'];
Try this:
// Some assoc list
var offers = {'x':{..some object...}, 'jjj':{...some other object ...}};
// First element (see attribution below)
return offers[Object.keys(offers)[0]];
// Last element (thanks to discussion on finding last element in associative array :)
return offers[Object.keys(offers)[Object.keys(offers).length - 1]];
Actually JavaScript doesn't support associative arrays, so you can't loop through it in an implied order (e.g. you can't access it via the indexer property array[0] won't access the first element in your object). The syntax is what makes it look like it does, but in reality it doesn't. So you have no "Order" to your objects.
http://www.hunlock.com/blogs/Mastering_Javascript_Arrays
Javascript does not have, and does not
support Associative Arrays. However…
All arrays in Javascript are objects
and Javascript's object syntax gives a
basic emulation of an associative
Array. For this reason the example
code above will actually work. Be
warned that this is not a real array
and it has real pitfals if you try to
use it. The 'person' element in the
example becomes part of the Array
object's properties and methods, just
like .length, .sort(), .splice(), and
all the other built-in properties and
methods.
Just thinking off the top of my head, but could you have another array with the key value pairs swapped?
So the answer would be arrayKeyValueReversed['20101102'] = '20910930923';
When you sort the array, use the first item (array[0]) as the key to get the value in the arrayKeyValueReversed.