why can't i assign one object's property to another property? - javascript

what could i do to assign the value of firstName's property to lastName's?
const user = {
firstName: "Someone",
lastName: this.firstName,
};
console.log(user.lastName);
// outputs undefined


Using a getter:
const user = {
firstName: "Someone",
get lastName() { return this.firstName; }
};
console.log(user.lastName);

Related

Destructing object in JS [duplicate]

Let's suppose I have the following object:
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
And that I want only the id and fullName.
I will do the following :
const { id, fullName } = user
Easy-peasy, right?
Now let's suppose that I want to do the destructuring based on the value of another variable called fields.
const fields = [ 'id', 'fullName' ]
Now my question is : How can I do destructuring based on an array of keys?
I shamelessly tried the following without success:
let {[{...fields}]} = user and let {[...fields]} = user. Is there any way that this could be done?
Thank you

It's not impossible to destructure with a dynamic key. To prevent the problem of creating dynamic variables (as Ginden mentioned) you need to provide aliases.
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
const fields = [ 'id', 'fullName' ];
const object = {};
const {[fields[0]]: id, [fields[1]]: fullName} = user;
console.log(id); // 42
console.log(fullName); // { firstName: "John", lastName: "Doe" }
To get around the problem of having to define static aliases for dynamic values, you can assign to an object's dynamic properties. In this simple example, this is the same as reverting the whole destructuring, though :)
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
const fields = [ 'id', 'fullName' ];
const object = {};
({[fields[0]]: object[fields[0]], [fields[1]]: object[fields[1]]} = user);
console.log(object.id); // 42
console.log(object.fullName); // { firstName: "John", lastName: "Doe" }
sources:
https://twitter.com/ydkjs/status/699845396084846592
https://github.com/getify/You-Dont-Know-JS/blob/master/es6%20%26%20beyond/ch2.md#not-just-declarations

Paul Kögel's answer is great, but I wanted to give a simpler example for when you need only the value of a dynamic field but don't need to assign it to a dynamic key.
let obj = {x: 3, y: 6};
let dynamicField = 'x';
let {[dynamicField]: value} = obj;
console.log(value);

Short answer: it's impossible and it won't be possible.
Reasoning behind this: it would introduce new dynamically named variables into block scope, effectively being dynamic eval, thus disabling any performance optimization. Dynamic eval that can modify scope in fly was always regarded as extremely dangerous and was removed from ES5 strict mode.
Moreover, it would be a code smell - referencing undefined variables throws ReferenceError, so you would need more boilerplate code to safely handle such dynamic scope.

As discussed before, you can't destruct into dynamically named variables in JavaScript without using eval.
But you can get a subset of the object dynamically, using reduce function as follows:
const destruct = (obj, ...keys) =>
keys.reduce((a, c) => ({ ...a, [c]: obj[c] }), {});
const object = {
color: 'red',
size: 'big',
amount: 10,
};
const subset1 = destruct(object, 'color');
const subset2 = destruct(object, 'color', 'amount', 'size');
console.log(subset1);
console.log(subset2);

You can't destruct without knowing the name of the keys or using an alias for named variables
// you know the name of the keys
const { id, fullName } = user;
// use an alias for named variables
const { [fields[0]]: id, [fields[1]]: fullName } = user;
A solution is to use Array.reduce() to create an object with the dynamic keys like this:
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
const fields = [ 'id', 'fullName', 'age' ];
const obj = fields.reduce((acc, k) => ({ ...acc, ...(user.hasOwnProperty(k) && { [k]: user[k] }) }), {});
for(let k in obj) {
console.log(k, obj[k]);
}

I believe that the above answers are intellectual and valid because all are given by pro developers. :). But, I have found a small and effective solution to destructure any objects dynamically. you can destructure them in two ways. But both ways are has done the same action.
Ex:
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
using "Object.key", "forEach" and "window" object.
Object.keys(user).forEach(l=>window[l]=user[l]);
Simply using Object. assign method.
Object.assign(window, user)
Output:
console.log(id, displayName, fullName)
// 42 jdoe {firstName: "John", lastName: "Doe"}
Anyway, I am a newbie in JS. So, don't take it as an offense if you found any misleading info in my answer.

how can i get age from this keyword?

const meDetails = {
firstName: 'arman',
lastName: 'soltani',
birthday: 1991,
hasDriverLicense: true,
calcAge: function () {
this.age = 2037 - this.birthday;
return this.age;
}
};
console.log(meDetails.age);
why the age is not defined??

the variable of named 'age' is not initialized before calling calcAge.
at first you should call calcAge to init meDetailes.age variable at runtime
then meDetails.age is valued and you can use it

You need to call calcAge() first to set the age property
meDetails.calcAge();
console.log(meDetails.age);
But you probably want a getter for age.
const meDetails = {
firstName: 'arman',
lastName: 'soltani',
birthday: 1991,
hasDriverLicense: true,
get age() {
return 2037 - this.birthday;
}
};
console.log(meDetails.age);

How can you define an objects properties as variables without knowing their names? [duplicate]

Let's suppose I have the following object:
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
And that I want only the id and fullName.
I will do the following :
const { id, fullName } = user
Easy-peasy, right?
Now let's suppose that I want to do the destructuring based on the value of another variable called fields.
const fields = [ 'id', 'fullName' ]
Now my question is : How can I do destructuring based on an array of keys?
I shamelessly tried the following without success:
let {[{...fields}]} = user and let {[...fields]} = user. Is there any way that this could be done?
Thank you

It's not impossible to destructure with a dynamic key. To prevent the problem of creating dynamic variables (as Ginden mentioned) you need to provide aliases.
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
const fields = [ 'id', 'fullName' ];
const object = {};
const {[fields[0]]: id, [fields[1]]: fullName} = user;
console.log(id); // 42
console.log(fullName); // { firstName: "John", lastName: "Doe" }
To get around the problem of having to define static aliases for dynamic values, you can assign to an object's dynamic properties. In this simple example, this is the same as reverting the whole destructuring, though :)
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
const fields = [ 'id', 'fullName' ];
const object = {};
({[fields[0]]: object[fields[0]], [fields[1]]: object[fields[1]]} = user);
console.log(object.id); // 42
console.log(object.fullName); // { firstName: "John", lastName: "Doe" }
sources:
https://twitter.com/ydkjs/status/699845396084846592
https://github.com/getify/You-Dont-Know-JS/blob/master/es6%20%26%20beyond/ch2.md#not-just-declarations

Paul Kögel's answer is great, but I wanted to give a simpler example for when you need only the value of a dynamic field but don't need to assign it to a dynamic key.
let obj = {x: 3, y: 6};
let dynamicField = 'x';
let {[dynamicField]: value} = obj;
console.log(value);

Short answer: it's impossible and it won't be possible.
Reasoning behind this: it would introduce new dynamically named variables into block scope, effectively being dynamic eval, thus disabling any performance optimization. Dynamic eval that can modify scope in fly was always regarded as extremely dangerous and was removed from ES5 strict mode.
Moreover, it would be a code smell - referencing undefined variables throws ReferenceError, so you would need more boilerplate code to safely handle such dynamic scope.

As discussed before, you can't destruct into dynamically named variables in JavaScript without using eval.
But you can get a subset of the object dynamically, using reduce function as follows:
const destruct = (obj, ...keys) =>
keys.reduce((a, c) => ({ ...a, [c]: obj[c] }), {});
const object = {
color: 'red',
size: 'big',
amount: 10,
};
const subset1 = destruct(object, 'color');
const subset2 = destruct(object, 'color', 'amount', 'size');
console.log(subset1);
console.log(subset2);

You can't destruct without knowing the name of the keys or using an alias for named variables
// you know the name of the keys
const { id, fullName } = user;
// use an alias for named variables
const { [fields[0]]: id, [fields[1]]: fullName } = user;
A solution is to use Array.reduce() to create an object with the dynamic keys like this:
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
const fields = [ 'id', 'fullName', 'age' ];
const obj = fields.reduce((acc, k) => ({ ...acc, ...(user.hasOwnProperty(k) && { [k]: user[k] }) }), {});
for(let k in obj) {
console.log(k, obj[k]);
}

I believe that the above answers are intellectual and valid because all are given by pro developers. :). But, I have found a small and effective solution to destructure any objects dynamically. you can destructure them in two ways. But both ways are has done the same action.
Ex:
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
using "Object.key", "forEach" and "window" object.
Object.keys(user).forEach(l=>window[l]=user[l]);
Simply using Object. assign method.
Object.assign(window, user)
Output:
console.log(id, displayName, fullName)
// 42 jdoe {firstName: "John", lastName: "Doe"}
Anyway, I am a newbie in JS. So, don't take it as an offense if you found any misleading info in my answer.

Lazy computed properties in Immutable.js Records?

Is there a standard way to define lazy computed properties on Records? When I access the computed property, it should run a function to compute the value, then cache the value. For example, something like:
const UserRecord = Record({
firstName: '',
lastName: '',
get fullName() {
console.log('Ran computation');
return `${this.firstName} ${this.lastName}`;
},
});
const user = new UserRecord({ firstName: 'Mark', lastName: 'Zuck' });
console.log(user.fullName); // Ran computation\nMark Zuck
console.log(user.fullName); // Mark Zuck
The closest I can get is defining a getFullName() method, then manually memoizing the computed value. I.e.:
getFullName() {
if (!this._fullName) {
this._fullName = `${this.firstName} ${this.lastName}`;
}
return this._fullName;
}

As long as you don't need it to ever "re-compute" this should do it I think.
const UserRecord = Record({
firstName: '',
lastName: '',
get fullName() {
console.log('Ran computation');
var value = `${this.firstName} ${this.lastName}`;
delete this.fullName;
Object.defineProperty(this, 'fullName', {
value: value
});
return value;
},
});
const user = new UserRecord({ firstName: 'Mark', lastName: 'Zuck' });
console.log(user.fullName); // Ran computation\nMark Zuck
console.log(user.fullName); // Mark Zuck

How to destructure into dynamically named variables in ES6?

Let's suppose I have the following object:
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
And that I want only the id and fullName.
I will do the following :
const { id, fullName } = user
Easy-peasy, right?
Now let's suppose that I want to do the destructuring based on the value of another variable called fields.
const fields = [ 'id', 'fullName' ]
Now my question is : How can I do destructuring based on an array of keys?
I shamelessly tried the following without success:
let {[{...fields}]} = user and let {[...fields]} = user. Is there any way that this could be done?
Thank you

It's not impossible to destructure with a dynamic key. To prevent the problem of creating dynamic variables (as Ginden mentioned) you need to provide aliases.
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
const fields = [ 'id', 'fullName' ];
const object = {};
const {[fields[0]]: id, [fields[1]]: fullName} = user;
console.log(id); // 42
console.log(fullName); // { firstName: "John", lastName: "Doe" }
To get around the problem of having to define static aliases for dynamic values, you can assign to an object's dynamic properties. In this simple example, this is the same as reverting the whole destructuring, though :)
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
const fields = [ 'id', 'fullName' ];
const object = {};
({[fields[0]]: object[fields[0]], [fields[1]]: object[fields[1]]} = user);
console.log(object.id); // 42
console.log(object.fullName); // { firstName: "John", lastName: "Doe" }
sources:
https://twitter.com/ydkjs/status/699845396084846592
https://github.com/getify/You-Dont-Know-JS/blob/master/es6%20%26%20beyond/ch2.md#not-just-declarations

Paul Kögel's answer is great, but I wanted to give a simpler example for when you need only the value of a dynamic field but don't need to assign it to a dynamic key.
let obj = {x: 3, y: 6};
let dynamicField = 'x';
let {[dynamicField]: value} = obj;
console.log(value);

Short answer: it's impossible and it won't be possible.
Reasoning behind this: it would introduce new dynamically named variables into block scope, effectively being dynamic eval, thus disabling any performance optimization. Dynamic eval that can modify scope in fly was always regarded as extremely dangerous and was removed from ES5 strict mode.
Moreover, it would be a code smell - referencing undefined variables throws ReferenceError, so you would need more boilerplate code to safely handle such dynamic scope.

As discussed before, you can't destruct into dynamically named variables in JavaScript without using eval.
But you can get a subset of the object dynamically, using reduce function as follows:
const destruct = (obj, ...keys) =>
keys.reduce((a, c) => ({ ...a, [c]: obj[c] }), {});
const object = {
color: 'red',
size: 'big',
amount: 10,
};
const subset1 = destruct(object, 'color');
const subset2 = destruct(object, 'color', 'amount', 'size');
console.log(subset1);
console.log(subset2);

You can't destruct without knowing the name of the keys or using an alias for named variables
// you know the name of the keys
const { id, fullName } = user;
// use an alias for named variables
const { [fields[0]]: id, [fields[1]]: fullName } = user;
A solution is to use Array.reduce() to create an object with the dynamic keys like this:
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
const fields = [ 'id', 'fullName', 'age' ];
const obj = fields.reduce((acc, k) => ({ ...acc, ...(user.hasOwnProperty(k) && { [k]: user[k] }) }), {});
for(let k in obj) {
console.log(k, obj[k]);
}

I believe that the above answers are intellectual and valid because all are given by pro developers. :). But, I have found a small and effective solution to destructure any objects dynamically. you can destructure them in two ways. But both ways are has done the same action.
Ex:
const user = {
id: 42,
displayName: "jdoe",
fullName: {
firstName: "John",
lastName: "Doe"
}
};
using "Object.key", "forEach" and "window" object.
Object.keys(user).forEach(l=>window[l]=user[l]);
Simply using Object. assign method.
Object.assign(window, user)
Output:
console.log(id, displayName, fullName)
// 42 jdoe {firstName: "John", lastName: "Doe"}
Anyway, I am a newbie in JS. So, don't take it as an offense if you found any misleading info in my answer.

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