Object.keys(obj) returns an Array of strings that are the keys of an object.
But what if the object is an array and I want the list of integer indexes that it has?
Is there a simple way to extract this without having to parseInt() them all?
Alternatively, is there a simple way to implement a sort of Object.values() to get an Array of the values (with normal Array integer keys) from an object?
You can loop the array for():
var arr = ["aaaa", "bbbb", "cccc"];
var iArr = [];
for(var i in arr)
{
iArr[i] = i;
alert(i+ " > " + arr[i]);
}
alert(iArr.length);
http://jsfiddle.net/Achilleterzo/kfLzD/
According to the final final final final draft of ES5, it seems that there is nothing like what you are looking for.
I think that all you can do is
var numericKeys = Object.keys(myObject).filter(function (key) {
return parseInt(key, 10).toString() === key;
});
Related
I have an array object where there are key value pairs. I am trying to get the keys in that array using a loop but I am getting only 0. What is the problem with my code.
var strj = '{"name":"John","age":"30","cars":
[ {"type":"car", "year":"1998"},
{"type":"van", "year":"1995"}]}';
var myobj = JSON.parse(strj)
var care = myobj.cars.filter(c => c.type=='car');
Value of care
0:{type: "car", year: "1998"}
length:1
__proto__:Array(0)
Loop
for (var key in care){
if(care.hasOwnProperty(key)){
console.log(key)
}
}
care is a array type so you cannot do for (var key in care). You need to do for (var key in care[0]). This is because for (var key in care) will look for the key value in care and since it is a array it will always take 0 as a value in key(as you have only one object in array and its index is 0). That is why you got 0 in console.log.
var care =[{type: "car", year: "1998"}];
for (var key in care[0]){
if(care[0].hasOwnProperty(key)){
console.log(key)
}
}
care.forEach( ( singleCar ) => {
for ( var key in singleCar ){
console.log(key);
if( care.hasOwnProperty( key ) ){
console.log(key);
}
}
})
forEach will give you all the objects one by one. so you can check them.
As others have solved the issue, might i make a suggestion - Object.keys () gives an array of the keys for a given object. Since you are getting your filtered object and simply want its keys - the following will achieve that. Note that this is only using the code after you have filtered the original and have gained the "care" object.
As an aside, note that object.values() will give you an array of the values in a given object and object.entries() will give you arrays of the key / value pairing.
var care = {type: "car", year: "1998"};
var keys = Object.keys(care)
console.log(keys) // gives ["type","year"]
filter() method returns a Array of matches.
var care = myobj.cars.filter(c => c.type=='car'); // So, this returns an array.
care.forEach(element => {
console.log(Object.keys(element)); //Prints keys of each element
});
Well actually there is no problem in your code at all. But you just misunderstood the use of javascript filter. Javascript filter() creates new array that's why you are getting 0 as key. If you want to get only one matching element then find() is what you should use.
var strj = '{"name":"John","age":"30","cars":[{"type":"car", "year":"1998"},{"type":"van", "year":"1995"}]}';
var myobj = JSON.parse(strj)
var care = myobj.cars.filter(c => c.type == 'car'); // returns array
var care = myobj.cars.find(c => c.type == 'car'); // returns first matching object
var care = myobj.cars.findIndex(c => c.type == 'car'); // returns first matching index
Javascript filter() method => Read Here
Javascript find() => Read Here
Javascript findIndex() method => Read Here
I am trying to parse a string in JS with a series of vars inline. The goal is to turn those vars into an object with name value pairs.
Example:
var hwStr = "color=blue+coreCnt=4+shell=aluminum+wireless=false";
I know I can parse the original string to get an array of name value pairs like this:
varArr = hwStr.split("+");
When I print that array I would get:
>color=blue,
>coreCnt=4,
>shell=aluminum,
>wireless=false
In order to create this object manually it would look like:
var hwSpec = {color: 'blue', coreCnt: 4, shell: 'aluminum', wireless: false};
My question is, how can I use a foreach statement to create an object that would have these as name value pairs.
To be fair JS is not my language, but I know that I SHOULD know this... This is probably a noob Question, any help would be great.
Gary C aka the UnKulMunki
After splitting on the plus signs, you can .reduce() the resulting array to process each key=value pair and add to an object:
var hwStr = "color=blue+coreCnt=4+shell=aluminum+wireless=false";
var obj = hwStr.split("+").reduce(function(o, item) {
item = item.split("=");
o[item[0]] = item[1];
return o;
}, {});
console.log(obj);
This is similar to using .forEach(), except instead of creating an empty object in a variable before calling .forEach() the empty object is passed as an argument to .reduce(). For this particular problem it doesn't make much difference, but in some cases .reduce() saves you having to create a temporary working variable.
EDIT: Note that my code creates all property values as strings - I don't think there's any way to know whether false should be treated as the boolean false or the string "false", unless you want to assume that all values that can be parsed as boolean or number should be treated as boolean or number.
First, you split the string at the + so you get an array of key/value pairs.
Then, you loop through those pairs and split each pair at the = to separate the key from the value. Then you assign the key as a property name and the value as the property value.
var hwStr = "color=blue+coreCnt=4+shell=aluminum+wireless=false";
// Split the string into an array of key/value pairs
var pairs = hwStr.split("+");
// Set up a new, empty object
var newObj = {};
// Loop through the key/value pairs array. The .forEach method takes
// a function as an argument that, itself, receives a value representing
// the current array item being iterated (a single key/value pair from
// the array in this case).
pairs.forEach(function(pair){
// Create a new property on the object with the key of the current pair
// and a value of the value of the current pair.
newObj[pair.split("=")[0]] = pair.split("=")[1];
});
console.log(newObj);
To do this, you have to use JSON's parse method to turn the string to javaScript object literal, this is how to do it:
var arr = hwStr.split("+");
var temp_arr = null;
var hwSpec = null;
var stringToConv = '{'; //string to convert to object literal
//iterate through the array
for (var i = 0; i < arr.length; i++){
temp_arr = arr[i].split("=");
stringToConv += '"' + temp_arr[0] + '":"' + temp_arr[1] + '"';
//check if is the last string in the arr
if (i === arr.length - 1){
stringToConv += '}'
}
else { //add comma
stringToConv += ",";
}
}
//convert to object using JSON
hwSpec = JSON.parse(stringToConv);
//your code here
window.onload = function() {
var arr = new Array;
var jsonObj = {
"123": "234"
};
arr['v'] = "234";
arr[0] = jsonObj;
arr[1] = jsonObj;
console.log(JSON.stringify(arr));
}
The above code result is :
[{"123":"234"},{"123":"234"}]
I don't know why the arr['v'] disappeared?
Object and Array are not parsed to JSON the same way.
an Array will only include the numeric keys, and an Object will include all of its keys:
var Arr = [], Obj ={};
Arr[0] = Obj[0] = 'a';
Arr[1] = Obj[2] = 'b';
Arr['key'] = Obj['key'] = 'c';
console.log(JSON.stringify(Arr));
console.log(JSON.stringify(Obj));
so in your case, you could simply use an Onject instead of an array:
var arr = new Object;
var jsonObj = {"123":"234"};
arr['v'] = "234";
arr[0] = jsonObj;
arr[1] = jsonObj;
console.log(JSON.stringify(arr));
Actually, JSON.stringify will ignore non-numeric keys in Array during the Array JSON serialization. From the latest ECMA Spec, in section "24.3.2.4 Runtime Semantics: SerializeJSONArray ( value )", we know JSON.stringify only utilizes length of Array and related numeric keys to do the serialization, and Array length will not be affected by its non-numeric keys. So it is now clear why 'v' (non-numeric keys) disappear in your final result.
You can't use a string as an array index, unless it is a string representation of an integer.
Therefore, arr['v'] has no meaning.
This stackoverflow question goes into more detail, but the relevant part:
Yes, technically array-indexes are strings, but as Flanagan elegantly
put it in his 'Definitive guide':
"It is helpful to clearly distinguish an array index from an object
property name. All indexes are property names, but only property names
that are integers between 0 and 232-1 are indexes."
In JavaScript, basically two types of array, Standard array and associative array. Standard array is defined by [], so 0 based type indexes. And in associative array is defined by {}, in this case you can define string as keys. So in your code you are using both is single array, that's not acceptable. So define the another array if you want strings keys.
I'm adding a bunch of input fields into an associative array. I can access the individual elements fine, eg. this works:
arr = new Array();
field = document.getElementById("someField");
arr[field] = someValue;
alert(arr[field].id);
But when I try to loop over them, the id shows up as undefined, and only one element is looped over.
for (var elem in arr) {
alert(elem.id + " " + arr[elem]);
}
Am I looping over it wrong?
Edit: arr.length shows up as 0 for some reason even though I'm able to access its elements.
the key in a javascript-array has to be a number or string.
field is automatically converted to a string with toString().
arr = new Array();
field = document.getElementById("someField");
var key = field.toString();
arr[key] = someValue;
alert(arr[key].id);
in your for-loop, you iterate the keys of that array.
field.toString() in that case.
and a string does not have a id-property.
this will work:
for (var elem in arr) {
alert(arr[elem].id + " " + arr[elem]);
}
by the way toString() of a DOM-Element ist often a generic string like "[SpanElement]".
if you try to add multiple span-elements, you're effectivle overriding the item with "[SpanElement]" as key and end up with just one element.
in respect to #user2736012 comments, i encourage everyone to read
"JavaScript Associative Arrays Demystified"
Any associative array in JavaScript is an object. Arrays are objects that have special methods because they are numerically indexed. So your code should look something like this:
obj = {};
field = document.getElementById("someField");
obj[field] = someValue;
for (var p in obj) {
alert(obj[p].id);
}
I got a json object in JavaScript like:
var json = {"20121207":"13", "20121211":"9", "20121213":"7", "20121219":"4"};
without knowing the name of the last key. (The keys are in ascending order)
How can I read the value (and key) of the last element?
var highest = json[ Object.keys(json).sort().pop() ];
Object.keys (ES5, shimmable) returns an array of the object's keys. We then sort them and grab the last one.
You can't ensure order in a for..in loop, so we can't completely rely on that. But as you said the keys are in ascending order, we can simply sort them.
Try this:
var lastKey;
var json = {"20121207":"13", "20121211":"9", "20121213":"7", "20121219":"4"};
for(var key in json){
if(json.hasOwnProperty(key)){
lastKey = key;
}
}
alert(lastKey + ': ' + json[lastKey]);
If you don't need to support old JS engines:
var lastKey = Object.keys(json).sort().reverse()[0];
var lastValue = json[lastKey];
Don't assume the keys are in order. Get the keys, and then sort them, and then grab that largest value after the sort.
Object keys are typically unordered, but looking at the keyset you are using, I made the assumption that you are looking for the highest date in the json keys:
var json = {"20121207":"13", "20121211":"9", "20121213":"7", "20121219":"4"};
function getMaxFromKeys(json) {
var m;
for (var i in json) {
if (json.hasOwnProperty(i)) {
m = (typeof m == 'undefined' || i > m) ? i : m;
}
}
return m;
}
var latestDate = getMaxFromKeys(json);
ECMCA script specifications (javascript specifications) do not require browsers to maintain order of the properties of an object.
Related to for-in this is what ECMCA 5 specs say:
"The mechanics and order of enumerating the properties (step 6.a in
the first algorithm, step 7.a in the second) is not specified."
Related to the Object.keys method:
"If an implementation defines a specific order of enumeration for the
for-in statement, that same enumeration order must be used in step 5
of this algorithm."
It does not make sense to say get the last defined property. You can sort the properties based on the name and get the last one, but if what you want is order then keep an array.
let obj = <your_object>
let last_key = Object.keys(obj)[Object.keys(obj).length - 1]
let last_value = <your_object>.last_key
OR
let last_value = Object.values(obj)[Object.values(obj).length - 1]
getJsonLastElement(json)
{
let len = json.length
return json[len - 1]
}