I'm wondering if there's a compact way to pull a property from an object, and then assign said property to the root of a new object, using the same property name.
Basically, I'd like to do the following without needing the first line:
const targetProp = someObj.data.targetProp;
const newObj = {
targetProp
}
What I'd imagine it might look like:
const newObj = {
[someObj.data.targetProp]
}
Where newObj would then have a property named 'targetProp', with the value of someObj.data.targetProp
There's no need for an extra variable:
const newObj = {
targetProp: someObj.data.targetProp
}
Destructuring is an option, which will reduce the size of the original code, but that requires keeping a first line:
const { targetProp } = someObj.data;
const newObj = { targetProp };
I don't think there's anything better than these two options.
I'm working on a function where I need to be able to input a string which is a key in a JSON object then I need to be able to take the actual object and tack on the string to get the correct value from the JSON
function contact(contact_method) {
let method = array[place].settings.contact_method; // Example for contact_method is 'first_contact_method'
console.log(method)
}
The idea is I have 3 different contact methods and I'd like to be able to use the same function for all 3. I know the code above is barely a function but I think it shows what I want to be able to do.
I could not find anything on MDN or SO about this. I had tried using ES6 and string with `` but that did not work it just returned [object Object].first_contact_method
You can access keys of objects with a variable by using [].
For instance:
const obj = { a: 4, b: 5, c: () => { /* do something*/}, d() { /* do something*/ } }
const keyA = 'a'
const keyC = 'c'
const valueA = obj[keyA] // valueA === 4
const methodC = obj[keyC]
// Call method c
methodC()
// or short
obj[keyC]()
// and even for "real" methods
obj['d']()
Is it possible to log variable name (not value) in JavaScript?
var max_value = 4;
console.log(max_value); // should log "max_value" as a string
UPDATE: I need a testing function that should be able to log any variable name (passed as an argument) as a string, not just this one variable.
There is a solution that can help you. I grabbed this function from this stackoverflow answer, which is able to get the name of the function parameters:
var STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;
var ARGUMENT_NAMES = /([^\s,]+)/g;
function getParamNames(func) {
var fnStr = func.toString().replace(STRIP_COMMENTS, '');
var result = fnStr.slice(fnStr.indexOf('(')+1, fnStr.indexOf(')')).match(ARGUMENT_NAMES);
if(result === null)
result = [];
return result;
}
then all you need to do now, is to use the name of your variable as a parameter of an anonymous function and pass all the function as argument of the getParamNames :
variablesNames = getParamNames(function (max_value, min_value) {});
This will return an array like this :
result => ["max_value", "min_value"];
Let's make it practical, first change the name of the getParamNames function to something easy and small like this :
function __ (func) {
// code here ...
}
second thing, instead of returning an array, just return the first element of the array, change this :
return result;
to this :
return result.shift();
now, you can get the name of your variable like this :
__(function( max_value ){});
I am aware of how to create getters and setters for properties whose names one already knows, by doing something like this:
// A trivial example:
function MyObject(val){
this.count = 0;
this.value = val;
}
MyObject.prototype = {
get value(){
return this.count < 2 ? "Go away" : this._value;
},
set value(val){
this._value = val + (++this.count);
}
};
var a = new MyObject('foo');
alert(a.value); // --> "Go away"
a.value = 'bar';
alert(a.value); // --> "bar2"
Now, my question is, is it possible to define sort of catch-all getters and setters like these? I.e., create getters and setters for any property name which isn't already defined.
The concept is possible in PHP using the __get() and __set() magic methods (see the PHP documentation for information on these), so I'm really asking is there a JavaScript equivalent to these?
Needless to say, I'd ideally like a solution that is cross-browser compatible.
This changed as of the ES2015 (aka "ES6") specification: JavaScript now has proxies. Proxies let you create objects that are true proxies for (facades on) other objects. Here's a simple example that turns any property values that are strings to all caps on retrieval, and returns "missing" instead of undefined for a property that doesn't exist:
"use strict";
if (typeof Proxy == "undefined") {
throw new Error("This browser doesn't support Proxy");
}
let original = {
example: "value",
};
let proxy = new Proxy(original, {
get(target, name, receiver) {
if (Reflect.has(target, name)) {
let rv = Reflect.get(target, name, receiver);
if (typeof rv === "string") {
rv = rv.toUpperCase();
}
return rv;
}
return "missing";
}
});
console.log(`original.example = ${original.example}`); // "original.example = value"
console.log(`proxy.example = ${proxy.example}`); // "proxy.example = VALUE"
console.log(`proxy.unknown = ${proxy.unknown}`); // "proxy.unknown = missing"
original.example = "updated";
console.log(`original.example = ${original.example}`); // "original.example = updated"
console.log(`proxy.example = ${proxy.example}`); // "proxy.example = UPDATED"
Operations you don't override have their default behavior. In the above, all we override is get, but there's a whole list of operations you can hook into.
In the get handler function's arguments list:
target is the object being proxied (original, in our case).
name is (of course) the name of the property being retrieved, which is usually a string but could also be a Symbol.
receiver is the object that should be used as this in the getter function if the property is an accessor rather than a data property. In the normal case this is the proxy or something that inherits from it, but it can be anything since the trap may be triggered by Reflect.get.
This lets you create an object with the catch-all getter and setter feature you want:
"use strict";
if (typeof Proxy == "undefined") {
throw new Error("This browser doesn't support Proxy");
}
let obj = new Proxy({}, {
get(target, name, receiver) {
if (!Reflect.has(target, name)) {
console.log("Getting non-existent property '" + name + "'");
return undefined;
}
return Reflect.get(target, name, receiver);
},
set(target, name, value, receiver) {
if (!Reflect.has(target, name)) {
console.log(`Setting non-existent property '${name}', initial value: ${value}`);
}
return Reflect.set(target, name, value, receiver);
}
});
console.log(`[before] obj.example = ${obj.example}`);
obj.example = "value";
console.log(`[after] obj.example = ${obj.example}`);
The output of the above is:
Getting non-existent property 'example'
[before] obj.example = undefined
Setting non-existent property 'example', initial value: value
[after] obj.example = value
Note how we get the "non-existent" message when we try to retrieve example when it doesn't yet exist, and again when we create it, but not after that.
Answer from 2011 (obsoleted by the above, still relevant to environments limited to ES5 features like Internet Explorer):
No, JavaScript doesn't have a catch-all property feature. The accessor syntax you're using is covered in Section 11.1.5 of the spec, and doesn't offer any wildcard or something like that.
You could, of course, implement a function to do it, but I'm guessing you probably don't want to use f = obj.prop("example"); rather than f = obj.example; and obj.prop("example", value); rather than obj.example = value; (which would be necessary for the function to handle unknown properties).
FWIW, the getter function (I didn't bother with setter logic) would look something like this:
MyObject.prototype.prop = function(propName) {
if (propName in this) {
// This object or its prototype already has this property,
// return the existing value.
return this[propName];
}
// ...Catch-all, deal with undefined property here...
};
But again, I can't imagine you'd really want to do that, because of how it changes how you use the object.
Preface:
T.J. Crowder's answer mentions a Proxy, which will be needed for a catch-all getter/setter for properties which don't exist, as the OP was asking for. Depending on what behavior is actually wanted with dynamic getters/setters, a Proxy may not actually be necessary though; or, potentially, you may want to use a combination of a Proxy with what I'll show you below.
(P.S. I have experimented with Proxy thoroughly in Firefox on Linux recently and have found it to be very capable, but also somewhat confusing/difficult to work with and get right. More importantly, I have also found it to be quite slow (at least in relation to how optimized JavaScript tends to be nowadays) - I'm talking in the realm of deca-multiples slower.)
To implement dynamically created getters and setters specifically, you can use Object.defineProperty() or Object.defineProperties(). This is also quite fast.
The gist is that you can define a getter and/or setter on an object like so:
let obj = {};
let val = 0;
Object.defineProperty(obj, 'prop', { //<- This object is called a "property descriptor".
//Alternatively, use: `get() {}`
get: function() {
return val;
},
//Alternatively, use: `set(newValue) {}`
set: function(newValue) {
val = newValue;
}
});
//Calls the getter function.
console.log(obj.prop);
let copy = obj.prop;
//Etc.
//Calls the setter function.
obj.prop = 10;
++obj.prop;
//Etc.
Several things to note here:
You cannot use the value property in the property descriptor (not shown above) simultaneously with get and/or set; from the docs:
Property descriptors present in objects come in two main flavors: data descriptors and accessor descriptors. A data descriptor is a property that has a value, which may or may not be writable. An accessor descriptor is a property described by a getter-setter pair of functions. A descriptor must be one of these two flavors; it cannot be both.
Thus, you'll note that I created a val property outside of the Object.defineProperty() call/property descriptor. This is standard behavior.
As per the error here, don't set writable to true in the property descriptor if you use get or set.
You might want to consider setting configurable and enumerable, however, depending on what you're after; from the docs:
configurable
true if and only if the type of this property descriptor may be changed and if the property may be deleted from the corresponding object.
Defaults to false.
enumerable
true if and only if this property shows up during enumeration of the properties on the corresponding object.
Defaults to false.
On this note, these may also be of interest:
Object.getOwnPropertyNames(obj): gets all properties of an object, even non-enumerable ones (AFAIK this is the only way to do so!).
Object.getOwnPropertyDescriptor(obj, prop): gets the property descriptor of an object, the object that was passed to Object.defineProperty() above.
obj.propertyIsEnumerable(prop);: for an individual property on a specific object instance, call this function on the object instance to determine whether the specific property is enumerable or not.
The following could be an original approach to this problem:
var obj = {
emptyValue: null,
get: function(prop){
if(typeof this[prop] == "undefined")
return this.emptyValue;
else
return this[prop];
},
set: function(prop,value){
this[prop] = value;
}
}
In order to use it the properties should be passed as strings.
So here is an example of how it works:
//To set a property
obj.set('myProperty','myValue');
//To get a property
var myVar = obj.get('myProperty');
Edit:
An improved, more object-oriented approach based on what I proposed is the following:
function MyObject() {
var emptyValue = null;
var obj = {};
this.get = function(prop){
return (typeof obj[prop] == "undefined") ? emptyValue : obj[prop];
};
this.set = function(prop,value){
obj[prop] = value;
};
}
var newObj = new MyObject();
newObj.set('myProperty','MyValue');
alert(newObj.get('myProperty'));
You can see it working here.
I was looking for something and I figured out on my own.
/*
This function takes an object and converts to a proxy object.
It also takes care of proxying nested objectsa and array.
*/
let getProxy = (original) => {
return new Proxy(original, {
get(target, name, receiver) {
let rv = Reflect.get(target, name, receiver);
return rv;
},
set(target, name, value, receiver) {
// Proxies new objects
if(typeof value === "object"){
value = getProxy(value);
}
return Reflect.set(target, name, value, receiver);
}
})
}
let first = {};
let proxy = getProxy(first);
/*
Here are the tests
*/
proxy.name={} // object
proxy.name.first={} // nested object
proxy.name.first.names=[] // nested array
proxy.name.first.names[0]={first:"vetri"} // nested array with an object
/*
Here are the serialised values
*/
console.log(JSON.stringify(first)) // {"name":{"first":{"names":[{"first":"vetri"}]}}}
console.log(JSON.stringify(proxy)) // {"name":{"first":{"names":[{"first":"vetri"}]}}}
var x={}
var propName = 'value'
var get = Function("return this['" + propName + "']")
var set = Function("newValue", "this['" + propName + "'] = newValue")
var handler = { 'get': get, 'set': set, enumerable: true, configurable: true }
Object.defineProperty(x, propName, handler)
this works for me
I am currently trying to do retrieve text from sqlite. I see that the amount of data requested do come correctly, but content, on the other hand, seems to be in an incorrect format. I've tried some conversion:
var data:Array = sqls.getResult().data;
var stData:String = String(data[0]);
Alert.show(stData); // <--- displays "[object Object]"
String conversion does not seem to do what I want. I simply want the text from the sqlite database. How can I convert the [object Object] to the correct string in this case?
Irrespective of what rows are returned (with(out)) specifying columns, unless the itemClass property of the SQLStatement is defined, it will always return an anonymous object. This is essentially how remoting works with AMF.
There are two things you can do (depending on the complexity of your project):
Specify an SQLStatement.itemClass - this will define & popuplate the results of the return with public accessors (either var or get/set) with the same name as the column name.
If left as anonymous - the column names are attached to the object in which you iterate the object just as you would if it was defined.
Super basic example:
//SQL table schema
CREATE TABLE accounts (
id INTEGER PRIMARY KEY AUTOINCREMENT,
num INTEGER NOT NULL,
name TEXT NOT NULL,
last_update DATE
);
//Define an Account class:
public class Account {
public var id:int;
public var num:int;
public var name:String;
public var last_update:Date;
}
//A statement to execute to get all accounts returned as an array of "Account"
var statement:SQLStatement = new SQLStatement();
statement.sqlConnection = myConn;
statement.itemClass = Account;
statement.text = 'SELECT * FROM accounts';
statement.addEventListener(SQLEvent.RESULT, onResults);
statement.execute();
protected function onResults(event:SQLEvent):void
{
var statement:SQLStatement = SQLStatement(event.currentTarget);
var results:SQLResult = statement.getResult();
for each (var account:Account in results)
{
//do something useful like populate a model, grid, etc...
}
}
//Anonymous objects would iterate much the same way -when an item class isn't defined
protected function onResults(event:SQLEvent):void
{
var statement:SQLStatement = SQLStatement(event.currentTarget);
var results:SQLResult = statement.getResult();
for each (var account:Object in results)
{
//our 'Object' will have properties: id, num, name, last_update
//do something useful like populate a model, grid, etc...
}
}
Please try with Objectutil.toString(); function which converts an object of any kind to a string.