While practising few examples , I encountered the following example :
var foo = {unique_prop: 1};
var bar = {unique_prop: 2};
var object = {};
object[foo] = 'value';
alert(object[bar]);
where two objects foo and bar are created . I am not getting how alert(object[bar]); is "value".
Whats the link here between foo and bar.
Also, a slight variation would give the output as "undefined" as the example below.
var foo = {unique_prop: 1};
var bar = {unique_prop: 2};
var object = {};
object["foo"] = 'value';
alert(object[bar]);
By default , the [] notation can use the strings right , arent ["some_property"] and [some_property] the same?
When using square bracket notation, anything inside the square brackets is converted into a string. Then that string is used to look for a property called the same thing.
var foo = {unique_prop: 1};
var bar = {unique_prop: 2};
var object = {};
object[foo] = 'value';
// foo is an object, so it's automatically turned into the string "[object Object]"
// so the above code is equivalent to `object["[object Object]"] = 'value';`
alert(object[bar]);
// bar is also an object, so is converted into the same string
// the above code is also equivalent to `alert(object["[object Object]"]);` which of course accesses that same value
var blah = "not blah";
object.blah = 1;
object["blah"] = 1;
object[blah];
// a variable is used.
// therefore the value of that variable is what the assessor is looking for, not the name of the variable.
// so the above code is equivalent to `object["not blah"];`.
An object's keys can only be strings*, so when you access an object's property using value which is not a string, it gets converted to a string.
In ECMAScript 6 you can use Map, which is similar to objects, but you can use any value as a key. Example:
const foo = {unique_prop: 1}
const bar = {unique_prop: 2}
const map = new Map()
map.set(foo, 'value')
console.log(map.get(bar)) // undefined
* In ECMAScript 6 you can also use symbols, but that's not relevant here.
Related
This question already has answers here:
Add a property to a JavaScript object using a variable as the name? [duplicate]
(14 answers)
Closed 7 years ago.
I want to add a new property to 'myObj', name it 'string1' and give it a value of 'string2', but when I do it it returns 'undefined:
var myObj = new Object;
var a = 'string1';
var b = 'string2';
myObj.a = b;
alert(myObj.string1); //Returns 'undefined'
alert(myObj.a); //Returns 'string2'
In other words: How do I create an object property and give it the name stored in the variable, but not the name of the variable itself?
There's the dot notation and the bracket notation
myObj[a] = b;
ES6 introduces computed property names, which allow you to do
var myObj = {[a]: b};
Dot notation and the properties are equivalent. So you would accomplish like so:
// const myObj = new Object();
const myObj = {};
const a = 'string1';
myObj[a] = 'whatever';
alert(myObj.string1);
(alerts "whatever")
Ecu, if you do myObj.a, then it looks for the property named a of myObj.
If you do myObj[a] =b then it looks for the a.valueOf() property of myObj.
Oneliner:
obj = (function(attr, val){ var a = {}; a[attr]=val; return a; })('hash', 5);
Or:
attr = 'hash';
val = 5;
var obj = (obj={}, obj[attr]=val, obj);
Anything shorter?
You could just use this:
function createObject(propName, propValue){
this[propName] = propValue;
}
var myObj1 = new createObject('string1','string2');
Anything you pass as the first parameter will be the property name, and the second parameter is the property value.
You cannot use a variable to access a property via dot notation, instead use the array notation.
var obj= {
'name' : 'jroi'
};
var a = 'name';
alert(obj.a); //will not work
alert(obj[a]); //should work and alert jroi'
As $scope is an object, you can try with JavaScript by:
$scope['something'] = 'hey'
It is equal to:
$scope.something = 'hey'
I created a fiddle to test.
The following demonstrates an alternative approach for returning a key pair object using the form of (a, b). The first example uses the string 'key' as the property name, and 'val' as the value.
Example #1:
(function(o,a,b){return o[a]=b,o})({},'key','val');
Example: #2:
var obj = { foo: 'bar' };
(function(o,a,b){return o[a]=b,o})(obj,'key','val');
As shown in the second example, this can modify existing objects, too (if property is already defined in the object, value will be overwritten).
Result #1: { key: 'val' }
Result #2: { foo: 'bar', key: 'val' }
This question already has answers here:
Add a property to a JavaScript object using a variable as the name? [duplicate]
(14 answers)
Closed 7 years ago.
I want to add a new property to 'myObj', name it 'string1' and give it a value of 'string2', but when I do it it returns 'undefined:
var myObj = new Object;
var a = 'string1';
var b = 'string2';
myObj.a = b;
alert(myObj.string1); //Returns 'undefined'
alert(myObj.a); //Returns 'string2'
In other words: How do I create an object property and give it the name stored in the variable, but not the name of the variable itself?
There's the dot notation and the bracket notation
myObj[a] = b;
ES6 introduces computed property names, which allow you to do
var myObj = {[a]: b};
Dot notation and the properties are equivalent. So you would accomplish like so:
// const myObj = new Object();
const myObj = {};
const a = 'string1';
myObj[a] = 'whatever';
alert(myObj.string1);
(alerts "whatever")
Ecu, if you do myObj.a, then it looks for the property named a of myObj.
If you do myObj[a] =b then it looks for the a.valueOf() property of myObj.
Oneliner:
obj = (function(attr, val){ var a = {}; a[attr]=val; return a; })('hash', 5);
Or:
attr = 'hash';
val = 5;
var obj = (obj={}, obj[attr]=val, obj);
Anything shorter?
You could just use this:
function createObject(propName, propValue){
this[propName] = propValue;
}
var myObj1 = new createObject('string1','string2');
Anything you pass as the first parameter will be the property name, and the second parameter is the property value.
You cannot use a variable to access a property via dot notation, instead use the array notation.
var obj= {
'name' : 'jroi'
};
var a = 'name';
alert(obj.a); //will not work
alert(obj[a]); //should work and alert jroi'
As $scope is an object, you can try with JavaScript by:
$scope['something'] = 'hey'
It is equal to:
$scope.something = 'hey'
I created a fiddle to test.
The following demonstrates an alternative approach for returning a key pair object using the form of (a, b). The first example uses the string 'key' as the property name, and 'val' as the value.
Example #1:
(function(o,a,b){return o[a]=b,o})({},'key','val');
Example: #2:
var obj = { foo: 'bar' };
(function(o,a,b){return o[a]=b,o})(obj,'key','val');
As shown in the second example, this can modify existing objects, too (if property is already defined in the object, value will be overwritten).
Result #1: { key: 'val' }
Result #2: { foo: 'bar', key: 'val' }
var foo = {prop:1};
var bar = {prop:2};
var obj = {};
obj[foo] = 'value';
console.log(obj[bar]); //'value'
It is quite confusing why obj[bar] and obj[foo] are same now, whereas foo and bar are not same.
because
obj[foo]='value';
is the same as
obj["[object Object]"]='value';
Object keys are string so it's internally doing foo.toString()
and foo.toString() === bar.toString()
Every objects will return "[object Object]" if used as a key (not talking about arrays, numbers, strings..)
You'll need something that uniquely identify your objects, like below
var foo = {id:"foo", prop:1};
var bar = {id:"bar", prop:2};
var obj = {};
obj[foo.id] = 'value';
console.log(obj[bar.id]); //undefined
I have an JavaScript object looking like this:
Object {#attributes: Object,…}
So how can I access the #attributes-Object?
parent["#attributes"], thus:
var parent = { "#attributes" : someObj} ;
console.log(parent["#attributes"]);
In JS, all property names can be used as named array elements. Most (i.e. those without spaces etc) can be used as bare property names.
var foo = { bar: 1};
foo.bar = 2 // or
foo["bar"] = 2
You can use the bracket notation:
var myObject = { '#attributes': 'foo' };
var result = myObject['#attributes']; // foo
I have an object in a variable var o={};
I want to do something like what .push() method doing in array for my object.
JS code:
// Array:
var ar=[];
ar.push('omid');
ar.push('F');
var got=ar[1];
// above code is standard but not what I'm looking for !
/*-------------------------------------*/
// Object:
var obj={};
/* obj.push('key','value'); // I want do something like this
var got2=obj.getVal('key'); // And this
*/
Is this possible at all ?
var obj = {}
// use this if you are hardcoding the key names
obj.key = 'value'
obj.key // => 'value'
// use this if you have strings with the key names in them
obj['key2'] = 'value'
obj['key2'] // => 'value'
// also use the second method if you have keys with odd names
obj.! = 'value' // => SyntaxError
obj['!'] = 'value' // => OK
Since Object-Literals use a Key->Value model, there is no JS method to "push" a value.
You can either use Dot Notation:
var Obj = {};
Obj.foo = "bar";
console.log(Obj);
Or Bracket Notation:
var Obj = {},
foo = "foo";
Obj[foo] = "bar";
Obj["bar"] = "foo";
console.log(Obj);
Consider reading https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Working_with_Objects, as arming yourself with this knowledge will be invaluable in the future.
Here is some javascript magic that makes it work.
Take a look.
var obj = {};
Object.defineProperty(obj,'push',{
value:function(x,y){
this[x]=y;
}
});
obj.push('name','whattttt'); <<<this works!!!
obj;
//{name:'whattttt'}
obj.name or obj['name']..
//whattttt
The reason i defined .push function using Object.defineProperty because i didn't want it to show up as a property of object. So if you have 3 items in object this would have always been the 4th one. And mess up the loops always. However, using this method. You can make properties hidden but accessible.
Though i don't know why you would use this method when there is already a easy way to do it.
to assign a value do this
obj.variable = 'value';
if value key is number or weird do this...
obj[1] = 'yes';
to access number or weird name you also do that
obj[1];
and finally to assign random keys or key that has been generated in code, not hard coded, than use this form too.
var person= 'him';
obj[him]='hello';