I'm trying to access a property of an object using a dynamic name. Is this possible?
const something = { bar: "Foobar!" };
const foo = 'bar';
something.foo; // The idea is to access something.bar, getting "Foobar!"
There are two ways to access properties of an object:
Dot notation: something.bar
Bracket notation: something['bar']
The value between the brackets can be any expression. Therefore, if the property name is stored in a variable, you have to use bracket notation:
var something = {
bar: 'foo'
};
var foo = 'bar';
// both x = something[foo] and something[foo] = x work as expected
console.log(something[foo]);
console.log(something.bar)
This is my solution:
function resolve(path, obj) {
return path.split('.').reduce(function(prev, curr) {
return prev ? prev[curr] : null
}, obj || self)
}
Usage examples:
resolve("document.body.style.width")
// or
resolve("style.width", document.body)
// or even use array indexes
// (someObject has been defined in the question)
resolve("part.0.size", someObject)
// returns null when intermediate properties are not defined:
resolve('properties.that.do.not.exist', {hello:'world'})
In javascript we can access with:
dot notation - foo.bar
square brackets - foo[someVar] or foo["string"]
But only second case allows to access properties dynamically:
var foo = { pName1 : 1, pName2 : [1, {foo : bar }, 3] , ...}
var name = "pName"
var num = 1;
foo[name + num]; // 1
// --
var a = 2;
var b = 1;
var c = "foo";
foo[name + a][b][c]; // bar
Following is an ES6 example of how you can access the property of an object using a property name that has been dynamically generated by concatenating two strings.
var suffix = " name";
var person = {
["first" + suffix]: "Nicholas",
["last" + suffix]: "Zakas"
};
console.log(person["first name"]); // "Nicholas"
console.log(person["last name"]); // "Zakas"
This is called computed property names
You can achieve this in quite a few different ways.
let foo = {
bar: 'Hello World'
};
foo.bar;
foo['bar'];
The bracket notation is specially powerful as it let's you access a property based on a variable:
let foo = {
bar: 'Hello World'
};
let prop = 'bar';
foo[prop];
This can be extended to looping over every property of an object. This can be seem redundant due to newer JavaScript constructs such as for ... of ..., but helps illustrate a use case:
let foo = {
bar: 'Hello World',
baz: 'How are you doing?',
last: 'Quite alright'
};
for (let prop in foo.getOwnPropertyNames()) {
console.log(foo[prop]);
}
Both dot and bracket notation also work as expected for nested objects:
let foo = {
bar: {
baz: 'Hello World'
}
};
foo.bar.baz;
foo['bar']['baz'];
foo.bar['baz'];
foo['bar'].baz;
Object destructuring
We could also consider object destructuring as a means to access a property in an object, but as follows:
let foo = {
bar: 'Hello World',
baz: 'How are you doing?',
last: 'Quite alright'
};
let prop = 'last';
let { bar, baz, [prop]: customName } = foo;
// bar = 'Hello World'
// baz = 'How are you doing?'
// customName = 'Quite alright'
You can do it like this using Lodash get
_.get(object, 'a[0].b.c');
UPDATED
Accessing root properties in an object is easily achieved with obj[variable], but getting nested complicates things. Not to write already written code I suggest to use lodash.get.
Example
// Accessing root property
var rootProp = 'rootPropert';
_.get(object, rootProp, defaultValue);
// Accessing nested property
var listOfNestedProperties = [var1, var2];
_.get(object, listOfNestedProperties);
Lodash get can be used in different ways, the documentation lodash.get
To access a property dynamically, simply use square brackets [] as follows:
const something = { bar: "Foobar!" };
const userInput = 'bar';
console.log(something[userInput])
The problem
There's a major gotchya in that solution! (I'm surprised other answers have not brought this up yet). Often you only want to access properties that you've put onto that object yourself, you don't want to grab inherited properties.
Here's an illustration of this issue. Here we have an innocent-looking program, but it has a subtle bug - can you spot it?
const agesOfUsers = { sam: 16, sally: 22 }
const username = prompt('Enter a username:')
if (agesOfUsers[username] !== undefined) {
console.log(`${username} is ${agesOfUsers[username]} years old`)
} else {
console.log(`${username} is not found`)
}
When prompted for a username, if you supply "toString" as a username, it'll give you the following message: "toString is function toString() { [native code] } years old". The issue is that agesOfUsers is an object, and as such, automatically inherits certain properties like .toString() from the base Object class. You can look here for a full list of properties that all objects inherit.
Solutions
Use a Map data structure instead. The stored contents of a map don't suffer from prototype issues, so they provide a clean solution to this problem.
const agesOfUsers = new Map()
agesOfUsers.set('sam', 16)
agesOfUsers.set('sally', 2)
console.log(agesOfUsers.get('sam')) // 16
Use an object with a null prototype, instead of the default prototype. You can use Object.create(null) to create such an object. This sort of object does not suffer from these prototype issues, because you've explicitly created it in a way that it does not inherit anything.
const agesOfUsers = Object.create(null)
agesOfUsers.sam = 16
agesOfUsers.sally = 22;
console.log(agesOfUsers['sam']) // 16
console.log(agesOfUsers['toString']) // undefined - toString was not inherited
You can use Object.hasOwn(yourObj, attrName) to first check if the dynamic key you wish to access is directly on the object and not inherited (learn more here). This is a relatively newer feature, so check the compatibility tables before dropping it into your code. Before Object.hasOwn(yourObj, attrName) came around, you would achieve this same effect via Object.prototype.hasOwnProperty.call(yourObj, attrName). Sometimes, you might see code using yourObj.hasOwnProperty(attrName) too, which sometimes works but it has some pitfalls that you can read about here.
// Try entering the property name "toString",
// you'll see it gets handled correctly.
const user = { name: 'sam', age: 16 }
const propName = prompt('Enter a property name:')
if (Object.hasOwn(user, propName)) {
console.log(`${propName} = ${user[propName]}`)
} else {
console.log(`${propName} is not found`)
}
If you know the key you're trying to use will never be the name of an inherited property (e.g. maybe they're numbers, or they all have the same prefix, etc), you can choose to use the original solution.
I came across a case where I thought I wanted to pass the "address" of an object property as data to another function and populate the object (with AJAX), do lookup from address array, and display in that other function. I couldn't use dot notation without doing string acrobatics so I thought an array might be nice to pass instead. I ended-up doing something different anyway, but seemed related to this post.
Here's a sample of a language file object like the one I wanted data from:
const locs = {
"audioPlayer": {
"controls": {
"start": "start",
"stop": "stop"
},
"heading": "Use controls to start and stop audio."
}
}
I wanted to be able to pass an array such as: ["audioPlayer", "controls", "stop"] to access the language text, "stop" in this case.
I created this little function that looks-up the "least specific" (first) address parameter, and reassigns the returned object to itself. Then it is ready to look-up the next-most-specific address parameter if one exists.
function getText(selectionArray, obj) {
selectionArray.forEach(key => {
obj = obj[key];
});
return obj;
}
usage:
/* returns 'stop' */
console.log(getText(["audioPlayer", "controls", "stop"], locs));
/* returns 'use controls to start and stop audio.' */
console.log(getText(["audioPlayer", "heading"], locs));
ES5 // Check Deeply Nested Variables
This simple piece of code can check for deeply nested variable / value existence without having to check each variable along the way...
var getValue = function( s, context ){
return Function.call( context || null, 'return ' + s )();
}
Ex. - a deeply nested array of objects:
a = [
{
b : [
{
a : 1,
b : [
{
c : 1,
d : 2 // we want to check for this
}
]
}
]
}
]
Instead of :
if(a && a[0] && a[0].b && a[0].b[0] && a[0].b[0].b && a[0].b[0].b[0] && a[0].b[0].b[0].d && a[0].b[0].b[0].d == 2 ) // true
We can now :
if( getValue('a[0].b[0].b[0].d') == 2 ) // true
Cheers!
Others have already mentioned 'dot' and 'square' syntaxes so I want to cover accessing functions and sending parameters in a similar fashion.
Code jsfiddle
var obj = {method:function(p1,p2,p3){console.log("method:",arguments)}}
var str = "method('p1', 'p2', 'p3');"
var match = str.match(/^\s*(\S+)\((.*)\);\s*$/);
var func = match[1]
var parameters = match[2].split(',');
for(var i = 0; i < parameters.length; ++i) {
// clean up param begninning
parameters[i] = parameters[i].replace(/^\s*['"]?/,'');
// clean up param end
parameters[i] = parameters[i].replace(/['"]?\s*$/,'');
}
obj[func](parameters); // sends parameters as array
obj[func].apply(this, parameters); // sends parameters as individual values
I asked a question that kinda duplicated on this topic a while back, and after excessive research, and seeing a lot of information missing that should be here, I feel I have something valuable to add to this older post.
Firstly I want to address that there are several ways to obtain the value of a property and store it in a dynamic Variable. The first most popular, and easiest way IMHO would be:
let properyValue = element.style['enter-a-property'];
however I rarely go this route because it doesn't work on property values assigned via style-sheets. To give you an example, I'll demonstrate with a bit of pseudo code.
let elem = document.getElementById('someDiv');
let cssProp = elem.style['width'];
Using the code example above; if the width property of the div element that was stored in the 'elem' variable was styled in a CSS style-sheet, and not styled inside of its HTML tag, you are without a doubt going to get a return value of undefined stored inside of the cssProp variable. The undefined value occurs because in-order to get the correct value, the code written inside a CSS Style-Sheet needs to be computed in-order to get the value, therefore; you must use a method that will compute the value of the property who's value lies within the style-sheet.
Henceforth the getComputedStyle() method!
function getCssProp(){
let ele = document.getElementById("test");
let cssProp = window.getComputedStyle(ele,null).getPropertyValue("width");
}
W3Schools getComputedValue Doc This gives a good example, and lets you play with it, however, this link Mozilla CSS getComputedValue doc talks about the getComputedValue function in detail, and should be read by any aspiring developer who isn't totally clear on this subject.
As a side note, the getComputedValue method only gets, it does not set. This, obviously is a major downside, however there is a method that gets from CSS style-sheets, as well as sets values, though it is not standard Javascript.
The JQuery method...
$(selector).css(property,value)
...does get, and does set. It is what I use, the only downside is you got to know JQuery, but this is honestly one of the very many good reasons that every Javascript Developer should learn JQuery, it just makes life easy, and offers methods, like this one, which is not available with standard Javascript.
Hope this helps someone!!!
For anyone looking to set the value of a nested variable, here is how to do it:
const _ = require('lodash'); //import lodash module
var object = { 'a': [{ 'b': { 'c': 3 } }] };
_.set(object, 'a[0].b.c', 4);
console.log(object.a[0].b.c);
// => 4
Documentation: https://lodash.com/docs/4.17.15#set
Also, documentation if you want to get a value: https://lodash.com/docs/4.17.15#get
You can do dynamically access the property of an object using the bracket notation. This would look like this obj[yourKey] however JavaScript objects are really not designed to dynamically updated or read. They are intended to be defined on initialisation.
In case you want to dynamically assign and access key value pairs you should use a map instead.
const yourKey = 'yourKey';
// initialise it with the value
const map1 = new Map([
['yourKey', 'yourValue']
]);
// initialise empty then dynamically assign
const map2 = new Map();
map2.set(yourKey, 'yourValue');
console.log(map1.get(yourKey));
console.log(map2.get(yourKey));
demo object example
let obj = {
name: {
first_name: "Bugs",
last_name: "Founder",
role: "Programmer"
}
}
dotted string key for getting the value of
let key = "name.first_name"
Function
const getValueByDottedKeys = (obj, strKey)=>{
let keys = strKey.split(".")
let value = obj[keys[0]];
for(let i=1;i<keys.length;i++){
value = value[keys[i]]
}
return value
}
Calling getValueByDottedKeys function
value = getValueByDottedKeys(obj, key)
console.log(value)
output
Bugs
const getValueByDottedKeys = (obj, strKey)=>{
let keys = strKey.split(".")
let value = obj[keys[0]];
for(let i=1;i<keys.length;i++){
value = value[keys[i]]
}
return value
}
let obj = {
name: {
first_name: "Bugs",
last_name: "Founder",
role: "Programmer"
}
}
let key = "name.first_name"
value = getValueByDottedKeys(obj, key)
console.log(value)
I bumped into the same problem, but the lodash module is limited when handling nested properties. I wrote a more general solution following the idea of a recursive descendent parser. This solution is available in the following Gist:
Recursive descent object dereferencing
Finding Object by reference without, strings,
Note make sure the object you pass in is cloned , i use cloneDeep from lodash for that
if object looks like
const obj = {data: ['an Object',{person: {name: {first:'nick', last:'gray'} }]
path looks like
const objectPath = ['data',1,'person',name','last']
then call below method and it will return the sub object by path given
const child = findObjectByPath(obj, objectPath)
alert( child) // alerts "last"
const findObjectByPath = (objectIn: any, path: any[]) => {
let obj = objectIn
for (let i = 0; i <= path.length - 1; i++) {
const item = path[i]
// keep going up to the next parent
obj = obj[item] // this is by reference
}
return obj
}
You can use getter in Javascript
getter Docs
Check inside the Object whether the property in question exists,
If it does not exist, take it from the window
const something = {
get: (n) => this.n || something.n || window[n]
};
You should use JSON.parse, take a look at https://www.w3schools.com/js/js_json_parse.asp
const obj = JSON.parse('{ "name":"John", "age":30, "city":"New York"}')
console.log(obj.name)
console.log(obj.age)
Person = (name, age, job) ->
#name = name
#age = age
#job = job
#sayName = ->
console.log(#name)
kim = new Person("Kimothy Lozado", 37, "Skip Tracer")
console.log kim.sayName()
Returned Error
TypeError: Object function () {
return console.log(this.name);
} has no method 'sayName'
at Object.<anonymous> (/Users/aero/Downloads/m.coffee:11:17)
at Object.<anonymous> (/Users/aero/Downloads/m.coffee:1:1)
at Module._compile (module.js:456:26)
Your not quite defining classes properly. Namely you need a constructor and need to use :s instead of =s unlike regular js.
Do this instead:
class Person
constructor: (#name, #age, #job) ->
sayName: ->
console.log(#name)
kim = new Person("Kimothy Lozado", 37, "Skip Tracer")
console.log kim.sayName()
Demo in a fiddle : http://jsfiddle.net/4e8xZ/
Here's a great article on CoffeeScript class from the little book of CoffeeScript for your reference.
I recommend agconti answer for creating classes, but here I want to answer to why this error occurs.
When you write a constructor in javascript:
function Person(name) {
this.name = name;
}
var someone = new Person('someone');
The variable someone is assigned to the returned object of the new keyword.
But if the constructor returns a function:
function Person(name) {
this.name = name;
return function(){};
}
var someone = new Person('someone');
Unfortunately, instead of getting the new created object, someone is assigned to the function returned by the constructor.
If we look at the compiled code of your coffee:
Person = function(name, age, job) {
this.name = name;
this.age = age;
this.job = job;
return this.sayName = function() {
return console.log(this.name);
};
};
We see that the constructor returns a function. this happens because coffescript always returns the last expression of a function.
You can fix it by returning nothing:
Person = (name, age, job) ->
#name = name
#age = age
#job = job
#sayName = ->
console.log(#name)
return
SO follow this example
example and how make a fake DBContex For test my test using just this work fine
[Test]
public void CiudadIndex()
{
var ciudades = new FakeDbSet<Ciudad>
{
new Ciudad {CiudadId = 1, EmpresaId =1, Descripcion ="Santa Cruz", FechaProceso = DateTime.Now, MarcaBaja = null, UsuarioId = 1},
new Ciudad {CiudadId = 2, EmpresaId =1, Descripcion ="La Paz", FechaProceso = DateTime.Now, MarcaBaja = null, UsuarioId = 1},
new Ciudad {CiudadId = 3, EmpresaId =1, Descripcion ="Cochabamba", FechaProceso = DateTime.Now, MarcaBaja = null, UsuarioId = 1}
};
//// Create mock unit of work
var mockData = new Mock<IContext>();
mockData.Setup(m => m.Ciudades).Returns(ciudades);
// Setup controller
var homeController = new CiudadController(mockData.Object);
// Invoke
var viewResult = homeController.Index();
var ciudades_de_la_vista = (IEnumerable<Ciudad>)viewResult.Model;
// Assert..
}
Iam tryign now to use Autofixture-Moq
to create "ciudades" but I cant. I try this
var fixture = new Fixture();
var ciudades = fixture.Build<FakeDbSet<Ciudad>>().CreateMany<FakeDbSet<Ciudad>>();
var mockData = new Mock<IContext>();
mockData.Setup(m => m.Ciudades).Returns(ciudades);
I get this error
Cant convert System.Collections.Generic.IEnumerable(FakeDbSet(Ciudad)) to System.Data.Entity.IDbSet(Ciudad)
cant put "<>" so I replace with "()" in the error message
Implementation of IContext and FakeDbSet
public interface IContext
{
IDbSet<Ciudad> Ciudades { get; }
}
public class FakeDbSet<T> : IDbSet<T> where T : class
how can make this to work?
A minor point... In stuff like:
var ciudades_fixture = fixture.Build<Ciudad>().CreateMany<Ciudad>();
The second type arg is unnecessary and should be:
var ciudades_fixture = fixture.Build<Ciudad>().CreateMany();
I really understand why you need a FakeDbSet and the article is a bit TL;DR... In general, I try to avoid faking and mucking with ORM bits and instead dealing with interfaces returning POCOs to the max degree possible.
That aside... The reason the normal syntax for initialising the list works is that there is an Add (and IEnumerable) in DBFixture. AutoFixture doesn't have a story for that pattern directly (after all it is compiler syntactic sugar and not particularly amenable to reflection or in line with any other conventions) but you can use AddManyTo as long as there is an ICollection in play. Luckily, within the impl of FakeDbSet as in the article, the following gives us an in:-
public ObservableCollection<T> Local
{
get { return _data; }
}
As ObservableCollection<T> derives from ICollection<T>, you should be able to:
var ciudades = new FakeDbSet<Cuidad>();
fixture.AddManyTo(ciudades.Local);
var mockData = new Mock<IContext>();
mockData.Setup(m => m.Ciudades).Returns(ciudades);
It's possible to wire up a customization to make this prettier, but at least you have a way to manage it. The other option is to have something implement ICollection (or add a prop with a setter taking IEnumerable<T> and have AF generate the parent object, causing said collection to be filled in.
Long superseded side note: In your initial question, you effectively have:
fixture.Build<FakeDbSet<Ciudad>>().CreateMany()
The problem becomes clearer then - you are asking AF to generate Many FakeDbSet<Ciudad>s, which is not what you want.
I haven't used AutoFixture in a while, but shouldn't it be:
var ciudades = new FakeDbSet<Ciudad>();
fixture.AddManyTo(ciudades);
for the moment I end doing this, I will keep reading about how use automoq, cause I'm new in this
var fixture = new Fixture();
var ciudades_fixture = fixture.Build<Ciudad>().CreateMany<Ciudad>();
var ciudades = new FakeDbSet<Ciudad>();
foreach (var item in ciudades_fixture)
{
ciudades.Add(item);
}
var mockData = new Mock<IContext>();
fixture.Create<Mock<IContext>>();
mockData.Setup(r => r.Ciudades).Returns(ciudades);
Answer:
It turns out I had neglected to use the new keyword when creating the class instance. The code in the question itself is fine.
Question:
I have a fairly simple class where the constructor calls another method on the class (editor_for_node). The call happens inside a jQuery each() loop, but I've also tried moving it outside.
define ['jquery'], ($) ->
class Editor
constructor: (#node, #data, #template) ->
#node.widgets().each (i, elem) =>
data = if #data then #data[i] else null
node = $(elem)
#editor_for_node node, data
editor_for_node: (node, data) ->
console.log 'hello!'
return {
'Editor': Editor,
}
When the line #editor_for_node node, data gets called, I get an error (in Firebug) saying this.editor_for_node is not a function.
I really can't see why this isn't working properly, the only possible source of weirdness that I can see is my use of require.js's define function at the start.
Edit: Generated output
(function() {
define(['jquery'], function($) {
var Editor;
Editor = (function() {
Editor.name = 'Editor';
function Editor(node, data, template) {
var _this = this;
this.node = node;
this.data = data;
this.template = template;
this.node.widgets().each(function(i, elem) {
data = _this.data ? _this.data[i] : null;
node = $(elem);
return _this.editor_for_node(node, data);
});
}
Editor.prototype.editor_for_node = function(node, data) {
return console.log('hello!');
};
return Editor;
})();
return {
'Editor': Editor
};
});
}).call(this);
First: Which version of CoffeeScript are you using? The fat arrow has been a source of bugs in certain previous releases.
If you're using the latest (1.3.1), then I'm going to go ahead and say that this is an indentation issue. When I copy and paste your code, it works fine. Are you mixing tabs and spaces? Verify that the compiled output contains the line
Editor.prototype.editor_for_node = ...
Update: See the comments on this answer. Turns out the problem was that the new keyword wasn't being used when invoking the constructor.