This works…
const { prop1:val1, prop2:val2 ) = req.query
val1 = val1.toLowerCase()
Though, I'm more inclined to do something like
const { prop1.toLowerCase():val1, prop2:val2 } = req.query
or
const { prop1:val1.toLowerCase(), prop2:val2 } = req.query
neither of which work. Is there a syntax similar to this or must manipulations be done outside of the destructing assignment?
No, this is not possible. A destructuring assignment does only assign, it does not do arbitrary transformations on the value. (Setters are an exception, but they would only complicate this).
I would recommend to write
const { prop1, prop2:val2 ) = req.query;
const val1 = prop1.toLowerCase();
or, in one statement:
const { prop1, prop2:val2 ) = req.query, val1 = prop1.toLowerCase();
The trouble with the temporary variable solutions is that they introduce different versions of the same data into the scope, which can lead to bugs.
This solution creates a utility function that receives the object to be destructured as well as a second object that is a mapping of property names to transformation functions. It's a little more verbose, but does the trick.
// Utility functions to perform specified transformations on an object
function transformProps(obj, trans) {
return Object.assign({}, obj, ...Object.entries(trans).map(([prop, fn]) =>
prop in obj ? {[prop]: fn(obj[prop])} : null
));
}
const { prop1:val1, prop2:val2 } = transformProps(
{prop1: "FOO", prop2: "BAR"},
{prop1: v => v.toLowerCase()} // Transformations to be made
);
console.log(val1, val2);
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)
I want to go through every key and get the name value from each key.
This is how my LocalStorage looks like.
key: 3 Value:
{"name":"Kevin","country":"Canada","about":"Test","image":""}
key: 4 Value:
{"name":"Homer","country":"Canada","about":"Test","image":""}
I want to getboth of these names and add them to my array. I tried it with this method:
for(var key in localStorage){
let user = JSON.parse(localStorage.getItem(key));
this.users.push(user);
}
Error I get is:
SyntaxError: Unexpected token e in JSON at position 1
var keys = Object.keys(localStorage);
keys.forEach(key=>{
var json_str =localStorage.getItem(key)
try {
var abc = JSON.parse(json_str);
this.user = abc;
} catch (e) {
console.log(e)
}
})
when you say I want to getboth of these names, i don't get it but either way you can try something like:
var keys = Object.keys(localStorage);
for(var i=0;i<keys.length;i++){
var key = keys[i];
console.log(key, localStorage[key]);
//store here "both names" where you want them
//you can also access each element with localStorage[key].name, localStorage[key].country, etc.
}
This is a refinement of Robert's answer.
Just enumerate all of the values (The keys themselves do not matter) in localStorage that have a name property that is a string. Then return that array.
Based on your own answer, you likely have inconsistent mutable state as moving your temporary variable to instance scope should not impact your situation.
function getUsers() {
return Object.values(localStorage)
.map(json => {
try {
return JSON.parse(json);
}
catch (e) {
return undefined;
}
})
.filter((user?: any): user is {name: string} => user && typeof user.name === 'string');
}
const users = getUsers();
You can consider using my library ngx-store to deal with localStorage, sessionStorage, cookies and a bit more in Angular. To achieve what you want you will be able to store whole array in storage or just use code like the below with your current data structure:
import { LocalStorageService } from 'ngx-store';
export class Example {
public users: Array<any> = [];
constructor(public localStorageService: LocalStorageService) {
this.localStorageService.utility.forEach((value, key) => this.users.push(value));
}
}
Really, it can be just that simple ;)
You can use method hasOwnProperty('propertyName') to check name available or not in object. Then perform the operation that you want.
let localStorage = {
"key1": {"name":"Kevin","country":"Canada","about":"Test","image":""},
"key2": {"name":"Homer","country":"Canada","about":"Test","image":""}
}
for(let key of Object.keys(localStorage)){
if(localStorage[key].hasOwnProperty('name')){
console.log(localStorage[key]['name']);
}
}
const allItems = []
const keys = Object.keys(window.localStorage); // all keys
keys.forEach(key=> {
const item = JSON.parse(localStorage.getItem(key) + ''); //item with type Object
allItems.push(item);
});
console.log(allItems) // arry of object
I manage to solve it, was a simple error by always initializing a new let user inside the loop.
I moved out the user and the rest of the code works.
user: any;
getUsers():void{
for(var key in localStorage){
this.user = JSON.parse(localStorage.getItem(key));
this.users.push(this.user);
}
}
How can I achieve the following using ImmutableJS:
myMap.get(key).push(newData);
You can do as follows: (see this JSBin)
const myMap = Immutable.fromJS({
nested: {
someKey: ['hello', 'world'],
},
});
const myNewMap = myMap.updateIn(['nested', 'someKey'], arr => arr.push('bye'));
console.log(myNewMap.toJS());
// {
// nested: {
// someKey: ["hello", "world", "bye"]
// }
// }
Since myMap is immutable, whenever you try to set/update/delete some data within it, it will return a reference to the new data. So, you would have to set it to a variable in order to access it (in this case, myNewMap).
If the array referenced at the key is a plain javascript array - then you will actually mutate that value - so your code will work as expected (ie - myMap will contain a mutable/mutated array at 'key' with the newData pushed in.) However, this kind of defeats the purpose of immutability so I would recommend that the key in myMap reference an Immutable.List. In which case you'll want to do:
var newMap = myMap.set('key', myMap.get('key').push(newData))