I have an object in NodeJS (a socket to be accurate).
When I print it, I see that one of the entries is this:
[Symbol(asyncId)]: 2781 // the numeric value changes
How can I obtain the value of such key?
I've tried socket['[Symbol(asyncId)]'] but got undefined.
The expression socket.[Symbol(asyncId)] would obviously not work.
You will not be able to access it directly by key, unless you have a reference to the actual: Symbol('asyncId'), because every Symbol is unique
The Symbol() function returns a value of type symbol, has static
properties that expose several members of built-in objects, has static
methods that expose the global symbol registry, and resembles a
built-in object class but is incomplete as a constructor because it
does not support the syntax "new Symbol()".
What you can do is loop through the object's own property keys, using Reflect.ownKeys, which will include normal properties & symbols, and then obtain that reference.
You can also use: Object.getOwnPropertySymbols()
function getObject() {
// You don't have access to this symbol, outside of this scope.
const symbol = Symbol('asyncId');
return {
foo: 'bar',
[symbol]: 42
};
}
const obj = getObject();
console.log(obj);
console.log(obj[Symbol('asyncId')]); // undefined
// or Object.getOwnPropertySymbols(obj)
const symbolKey = Reflect.ownKeys(obj)
.find(key => key.toString() === 'Symbol(asyncId)')
console.log(obj[symbolKey]); // 42
NOTE: The object can have multiple keys where key.toString() === 'Symbol(asyncId)', this won't be usual, but be aware, so you may want to use other function other than .find if that's the case.
NOTE II:
You should not change the value of that property, since it's supposed to be for internal access only, even if the property is not read only.
function getObject() {
// You don't have access to this symbol, outside of this scope.
const symbol = Symbol('asyncId');
const symbol2 = Symbol('asyncId');
return {
foo: 'bar',
[symbol]: 'the value I don\'t want',
[symbol2]: 'the value I want'
};
}
const obj = getObject();
const symbolKey = Reflect.ownKeys(obj)
.find(key => key.toString() === 'Symbol(asyncId)')
console.log(obj[symbolKey]); // the value I don't want
console.log('=== All values ===');
Reflect.ownKeys(obj)
.forEach(key => console.log(obj[key]));
Use Object.getOwnPropertySymbols(obj) and iterate over it
You need to store the Symbol in advance and use it as accessor for the object.
Every symbol value returned from Symbol() is unique. A symbol value may be used as an identifier for object properties; this is the data type's only purpose. Some further explanation about purpose and usage can be found in the glossary entry for Symbol.
var asyncId = 42,
symbol = Symbol(asyncId),
object = { [symbol]: 2781 };
console.log(object[symbol]);
console.log(symbol.toString());
Error I'm getting is this:
TypeError: Cannot read property 'latitude' of undefined
Here's the piece that's messing me up
const [user] = useCurrentUser();
var location = [user.latitude, user.longitude];
useCurrentUser() is here:
export function useCurrentUser() {
const { data, mutate } = useSWR('/api/user', fetcher);
const user = data?.user;
return [user, { mutate }];
}
I'm assuming I'm just calling it early, because it hasn't had a chance to go through the useCurrentUser() function yet. However, I can't for the life of me figure out how to call it when that's done?
edit:
It's also not usable later on in a component:
This works:
<span className="bold">{user ? user.latitude : ''}</span>
This doesn't:
<Map location={[user.longitude, user.latitude]}/>
What am I not understanding, hahaha
I am getting below error .
Error: Argument "data" is not a valid Document. Input is not a plain JavaScript object.
at Object.exports.(anonymous function) [as isDocument] (/user_code/node_modules/firebase-admin/node_modules/#google-cloud/firestore/src/validate.js:86:15)
at WriteBatch.set (/user_code/node_modules/firebase-admin/node_modules/#google-cloud/firestore/src/write-batch.js:286:14)
at DocumentReference.set (/user_code/node_modules/firebase-admin/node_modules/#google-cloud/firestore/src/reference.js:420:8)
at PainEntryService.createEntry (/user_code/services/entry-service.js:19:30)
at /user_code/services/intentService.js:22:36
at Function.<anonymous> (/user_code/node_modules/actions-on-google/dist/service/dialogflow/dialogflow.js:146:23)
at next (native)
at /user_code/node_modules/actions-on-google/dist/service/dialogflow/dialogflow.js:22:71
at __awaiter (/user_code/node_modules/actions-on-google/dist/service/dialogflow/dialogflow.js:18:12)
at Function.handler (/user_code/node_modules/actions-on-google/dist/service/dialogflow/dialogflow.js:84:16)
I am using below code for creating the document
var docRef = this.dbManager.collection('tetst).doc(subjectId);
var setAlan = docRef.set(EntryEntity.toJSON);
I have below code in the toJSON method
public toJSON(): testEntry {
let returnJSON = {
"entry_id": this.entry_id,
"subject_id": this.subject_id,
"entry_date": this.entry_date,
"questionnaire": this.questionnaire,
"entry_start_timestamp": this.entry_start_timestamp,
"entry_end_timestamp": this.entry_end_timestamp,
"entry_complete": this.entry_complete,
//"responses": this.responses,
"last_answered_question" : this.last_answered_question,
"entry_status" : this.entry_status
}
return returnJSON;
}
I am framing json object in the above method. If i print the json object, i am getting below content
{ entry_id: 'df2b4ad4-6a70-4304-a71f-3a63773ada61',
subject_id: 'ABwppHHkzfY1Whp-lCHNnvEcuqvsbMKtZsg_ui9vc4jtpXSiAbh0fNsg6LxGkYq-Va3SOrwcvD-HAs7VQA',
entry_date: '2018-06-15',
questionnaire: 1,
entry_start_timestamp: '2018-06-15T09:38:10.266Z',
entry_end_timestamp: '2018-06-15T09:38:10.266Z',
entry_complete: false,
last_answered_question: 0,
entry_status: 'active' }
How to solve above issue? is there any issue with json object?
You need to call the toJSON method. Currently you are passing a reference to a method to set when you need to pass the return value of the called method.
Change
var setAlan = docRef.set(EntryEntity.toJSON);
to
var setAlan = docRef.set(EntryEntity.toJSON());
I have these two blocks of code.
socket.on('chatMessage', function(message) {
message.type = 'message';
message.created = Date.now();
message.username : socket.request.user.username;
});
socket.on('disconnect', function() {
io.emit('chatMessage', {
type: 'status',
text: socket.request.user.username + ' left the conversation.',
created: Date.now(),
username: socket.request.user.username,
});
});
If i change : or = to other, Webstorm gives me error.Expression statement is not assignment or call. Could someone explain why this happens? Thanks in advance.
Your first block of code is defining a function block. Your second block of code is defining an object definition. There is different syntax allowed for each and you have to use the appropriate syntax to match the context.
Your first block of code is just executing a series of statements in a function block. The { at the end of this line:
socket.on('chatMessage', function(message) {
defines the beginning of a function block. Thus, Javascript is looking for statements that are legal in that context. Each line of code there needs to be a legal Javascript statement. And,
message.username : socket.request.user.username;
is not a legal statement in that context. If you were trying to assign a value to message.username, then the valid syntax for that would be:
message.username = socket.request.user.username;
In your second block, you are in the middle of an object definition. The { at the end of this line:
io.emit('chatMessage', {
starts an object definition. As such, the legal syntax for defining a property on that object looks like this:
username: socket.request.user.username,
So, there are two keys here:
Recognizing when a { signifies the start of a function block vs. the start of an object definition. Your first code block starts a function body, the second starts an object definition.
Knowing the difference between syntax allowed in a function block and syntax allowed in an object definition.
This is ok:
message.type = 'message';
This is ok:
message.created = Date.now();
This is not:
message.username : socket.request.user.username;
It should be:
message.username = socket.request.user.username;
Your question is not clear - do you get the warning when you change the : to = or before you change it. You didn't say which line causes the error, I assumed that it was the one with : instead of =.
Are you sure that you're not using TypeScript that could interpret the colons differently?
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