This question already has answers here:
Self-references in object literals / initializers
(30 answers)
Closed 5 years ago.
I am trying to create a JSON object to store some parameters for a program. Some of the parameters need to be calculated from others as they are being defined. I would like to do this within the object definition but maybe this is not possible
var params = {
a: 50,
b: 70,
c: this.a+this.b
}
Result
What happens
>params.c
NaN
What I wished happened
>params.c
120
Edit
After doing some further reading, I think I am using Object Literal Notation instead of JSON.
You can use this approach:
To avoid re-calculation, use the function Object.assign.
The get syntax binds an object property to a function that will be called when that property is looked up.
var params = {
a: 50,
b: 70,
get c() {
console.log('Called!');
return this.a + this.b;
}
};
console.log(params.c); // Prints 120 then Called!
console.log(params.c); // Prints 120 then Called!
var params = Object.assign({}, {
a: 50,
b: 70,
get c() {
console.log('Called from function Object.assign!');
return this.a + this.b;
}
});
params.a = 1000; // To illustrate.
console.log(params.c); // Prints 120
console.log(params.c); // Prints 120
.as-console-wrapper {
max-height: 100% !important
}
Resources
Object initializer
getter
Personally, I would create constants (since magic numbers are the devil), but this is an overly-simplistic example:
const FIFTY = 50;
const SEVENTY = 70;
var params = {
a: FIFTY,
b: SEVENTY,
c: FIFTY + SEVENTY
};
What I would recommend doing is starting with an object that does not contain c, and taking the calculation outside of the object. Once the calculation has occurred, simply add the sum back to the object as a new key/value pair:
var params = {
a: 50,
b: 70,
}
var sum = 0;
for (var el in params) {
if (params.hasOwnProperty(el)) {
sum += params[el];
}
}
params['c'] = sum;
console.log(params);
Related
This question already has answers here:
Destructure object parameter, but also have reference to the parameter as an object? [duplicate]
(2 answers)
Closed 3 years ago.
Is it possible to both destructure a variable and have access to the structured variable in the same call?
For example, what could I replace ??? below to get the desired output (and how else might I have to edit my code)
const foo = ({ a, b }) => {
console.log(a) // 1
console.log(b) // 2
console.log(???) // { a: 1, b: 2 }
}
const x = { a: 1, b: 2 }
foo(x)
My goal is knowledge and succinct code - I want to avoid const { a, b } = params as the first line of foo() in the case where I might need to pass the entire params object on.
If the first argument is an object whose reference you want, it's not possible - once you destructure an argument, there's no longer any way to reference to the full argument, only some of its properties.
It would be possible if the object and parts you wanted was part of a larger object, though, because then you can reference the property name alone (to get the object into a variable), and then reference it again to destructure, eg:
const foo = ({ obj: { a, b }, obj }) => {
console.log(a) // 1
console.log(b) // 2
console.log(obj) // { a: 1, b: 2 }
}
const obj = { a: 1, b: 2 }
foo({ obj })
Your original code could work to an extent if the object had a property that referenced itself, but that's pretty weird:
const foo = ({ a, b, obj }) => {
console.log(a) // 1
console.log(b) // 2
console.log(obj) // { a: 1, b: 2 }
}
const x = { a: 1, b: 2 }
x.obj = x;
foo(x)
This question already has answers here:
Self-references in object literals / initializers
(30 answers)
Closed 8 years ago.
I am trying to do something like this:
var obj = {
a: 5,
b: this.a + 1
}
(instead of 5 there is a function which I don't want to execute twice that returns a number)
I can rewrite it to assign obj.b later from obj.a, but can I do it right away during declaration?
No. this in JavaScript does not work like you think it does. this in this case refers to the global object.
There are only 3 cases in which the value this gets set:
The Function Case
foo();
Here this will refer to the global object.
The Method Case
test.foo();
In this example this will refer to test.
The Constructor Case
new foo();
A function call that's preceded by the new keyword acts as a constructor. Inside the function this will refer to a newly
created Object.
Everywhere else, this refers to the global object.
There are several ways to accomplish this; this is what I would use:
function Obj() {
this.a = 5;
this.b = this.a + 1;
// return this; // commented out because this happens automatically
}
var o = new Obj();
o.b; // === 6
This should return the correct values:
function () {
var aVar = 5;
var bVar = aVar + 1;
return {
a : aVar,
b : bVar;
}
}();
As it turns out you can't reference an object inside another object unless the first one is a function. But you can do it this way.
var obj = {
a: 5
}
obj.b = obj.a + 1; // create field b in runtime and assign it's value
If you console.log(obj) you will have
obj = {
a: 5,
b: 6
}
This way you keep the object literal structure for the remaining part of the code
No, in your example, the value of this doesn't refer to the object literal.
You'll need to assign a value to b after the object has been created in order to base it on another property in obj.
in chrome debugger
> var o = {a: 5, b: this.a+1}
undefined
> o.b
NaN
> o.a
5
No. this will take the same meaning as it would outside the definition.
Assume code of the following kind (e.g. using lodash or explicitly like here):
function extend(base, overwrite) {
for (var key in overwrite)
base[key] = overwrite[key];
return base;
}
var first = extend({
a: 1
}, {
b: 2
});
var second = extend({
c: 3
}, {
d: 4
});
console.log(first.a + first.b + second.c + second.d);
How can I explain to Flowtype that this is actually fine?
Try putting this just above function declaration:
declare function extend<A, B>(a: A, b: B): A & B
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Self-references in object literal declarations
var obj = {
value: 10,
value2: value + 2
};
How can I do the above? (Assuming it's even possible)
I am using a lot of jQuery's $.extend to add more properties that rely on the previous values being added; so that modifying a few values automatically correct the rest.
You can't do it in declaration, but you could always make a function. The following will always return a value 2 greater than that stored in value:
var obj = {
value: 10,
getValue2: function(){ return this.value + 2; }
};
If you don't want the value of value2 to change along with value, you could declare a placeholder variable and declare the object using that:
var placeholder = 10;
var obj = {
value: placeholder,
value2: placeholder + 2
};
I am not 100% sure if you can do this at the time of declaration. You could, however, do it afterwards:
using extend:
var obj = {
value: 10
};
$.extend(obj, {
value2: obj.value + 2
});
http://jsfiddle.net/KgKgf/
just javascript:
var obj = {
value: 10
};
obj.value2 = obj.value + 2;
http://jsfiddle.net/KgKgf/1/
This question already has answers here:
Self-references in object literals / initializers
(30 answers)
Closed 8 years ago.
I am trying to do something like this:
var obj = {
a: 5,
b: this.a + 1
}
(instead of 5 there is a function which I don't want to execute twice that returns a number)
I can rewrite it to assign obj.b later from obj.a, but can I do it right away during declaration?
No. this in JavaScript does not work like you think it does. this in this case refers to the global object.
There are only 3 cases in which the value this gets set:
The Function Case
foo();
Here this will refer to the global object.
The Method Case
test.foo();
In this example this will refer to test.
The Constructor Case
new foo();
A function call that's preceded by the new keyword acts as a constructor. Inside the function this will refer to a newly
created Object.
Everywhere else, this refers to the global object.
There are several ways to accomplish this; this is what I would use:
function Obj() {
this.a = 5;
this.b = this.a + 1;
// return this; // commented out because this happens automatically
}
var o = new Obj();
o.b; // === 6
This should return the correct values:
function () {
var aVar = 5;
var bVar = aVar + 1;
return {
a : aVar,
b : bVar;
}
}();
As it turns out you can't reference an object inside another object unless the first one is a function. But you can do it this way.
var obj = {
a: 5
}
obj.b = obj.a + 1; // create field b in runtime and assign it's value
If you console.log(obj) you will have
obj = {
a: 5,
b: 6
}
This way you keep the object literal structure for the remaining part of the code
No, in your example, the value of this doesn't refer to the object literal.
You'll need to assign a value to b after the object has been created in order to base it on another property in obj.
in chrome debugger
> var o = {a: 5, b: this.a+1}
undefined
> o.b
NaN
> o.a
5
No. this will take the same meaning as it would outside the definition.