In JavaScript, how to conditionally add a member to an object?
JavascriptJavascript Problem Overview
I would like to create an object with a member added conditionally. The simple approach is:
var a = {};
if (someCondition)
a.b = 5;
Now, I would like to write a more idiomatic code. I am trying:
a = {
b: (someCondition? 5 : undefined)
};
But now, b is a member of a whose value is undefined. This is not the desired result.
Is there a handy solution?
Update
I seek for a solution that could handle the general case with several members.
a = {
b: (conditionB? 5 : undefined),
c: (conditionC? 5 : undefined),
d: (conditionD? 5 : undefined),
e: (conditionE? 5 : undefined),
f: (conditionF? 5 : undefined),
g: (conditionG? 5 : undefined),
};
Javascript Solutions
Solution 1 - Javascript
I think @InspiredJW did it with ES5, and as @trincot pointed out, using es6 is a better approach. But we can add a bit more sugar, by using the spread operator, and logical AND short circuit evaluation:
const a = {
...(someCondition && {b: 5})
}
Solution 2 - Javascript
const obj = {
...(condition) && {someprop: propvalue},
...otherprops
}
Live Demo:
const obj = {
...(true) && {someprop: 42},
...(false) && {nonprop: "foo"},
...({}) && {tricky: "hello"},
}
console.log(obj);
Solution 3 - Javascript
In pure Javascript, I cannot think of anything more idiomatic than your first code snippet.
If, however, using the jQuery library is not out of the question, then $.extend() should meet your requirements because, as the documentation says:
> Undefined properties are not copied.
Therefore, you can write:
var a = $.extend({}, {
b: conditionB ? 5 : undefined,
c: conditionC ? 5 : undefined,
// and so on...
});
And obtain the results you expect (if conditionB is false, then b will not exist in a).
Solution 4 - Javascript
I suggest the following:
const a = {
...(someCondition? {b: 5}: {})
}
Solution 5 - Javascript
With EcmaScript2015 you can use Object.assign:
Object.assign(a, conditionB ? { b: 1 } : null,
conditionC ? { c: 2 } : null,
conditionD ? { d: 3 } : null);
var a, conditionB, conditionC, conditionD;
conditionC = true;
a = {};
Object.assign(a, conditionB ? { b: 1 } : null,
conditionC ? { c: 2 } : null,
conditionD ? { d: 3 } : null);
console.log(a);
Some remarks:
Object.assignmodifies the first argument in-place, but it also returns the updated object: so you can use this method in a bigger expression that further manipulates the object.- Instead of
nullyou could passundefinedor{}, with the same result. You could even provide0instead, because primitive values are wrapped, andNumberhas no own enumerable properties.
Even more concise
Taking the second point further, you could shorten it as follows (as @Jamie has pointed out), as falsy values have no own enumerable properties (false, 0, NaN, null, undefined, '', except document.all):
Object.assign(a, conditionB && { b: 1 },
conditionC && { c: 2 },
conditionD && { d: 3 });
var a, conditionB, conditionC, conditionD;
conditionC = "this is truthy";
conditionD = NaN; // falsy
a = {};
Object.assign(a, conditionB && { b: 1 },
conditionC && { c: 2 },
conditionD && { d: 3 });
console.log(a);
Solution 6 - Javascript
Conditionally Add a member to an Object
const trueCondition = true;
const falseCondition = false;
const obj = {
...(trueCondition && { student: 10 }),
...(falseCondition && { teacher: 2 }),
};
// { student: 10 }
Solution 7 - Javascript
Perfomance test
Classic approach
const a = {};
if (someCondition)
a.b = 5;
VS
spread operator approach
const a2 = {
...(someCondition && {b: 5})
}
Results:
The classic approach is much faster, so take in consideration that the syntax sugaring is slower.
> testClassicConditionFulfilled(); // ~ 234.9ms
> testClassicConditionNotFulfilled(); // ~493.1ms
> testSpreadOperatorConditionFulfilled(); // ~2649.4ms
> testSpreadOperatorConditionNotFulfilled(); // ~2278.0ms
function testSpreadOperatorConditionFulfilled() {
const value = 5;
console.time('testSpreadOperatorConditionFulfilled');
for (let i = 0; i < 200000000; i++) {
let a = {
...(value && {b: value})
};
}
console.timeEnd('testSpreadOperatorConditionFulfilled');
}
function testSpreadOperatorConditionNotFulfilled() {
const value = undefined;
console.time('testSpreadOperatorConditionNotFulfilled');
for (let i = 0; i < 200000000; i++) {
let a = {
...(value && {b: value})
};
}
console.timeEnd('testSpreadOperatorConditionNotFulfilled');
}
function testClassicConditionFulfilled() {
const value = 5;
console.time('testClassicConditionFulfilled');
for (let i = 0; i < 200000000; i++) {
let a = {};
if (value)
a.b = value;
}
console.timeEnd('testClassicConditionFulfilled');
}
function testClassicConditionNotFulfilled() {
const value = undefined;
console.time('testClassicConditionNotFulfilled');
for (let i = 0; i < 200000000; i++) {
let a = {};
if (value)
a.b = value;
}
console.timeEnd('testClassicConditionNotFulfilled');
}
testClassicConditionFulfilled(); // ~ 234.9ms
testClassicConditionNotFulfilled(); // ~493.1ms
testSpreadOperatorConditionFulfilled(); // ~2649.4ms
testSpreadOperatorConditionNotFulfilled(); // ~2278.0ms
Solution 8 - Javascript
more simplified,
const a = {
...(condition && {b: 1}) // if condition is true 'b' will be added.
}
Solution 9 - Javascript
What about using Enhanced Object Properties and only set the property if it is truthy, e.g.:
[isConditionTrue() && 'propertyName']: 'propertyValue'
So if the condition is not met it doesn't create the preferred property and thus you can discard it. See: http://es6-features.org/#ComputedPropertyNames
UPDATE: It is even better to follow the approach of Axel Rauschmayer in his blog article about conditionally adding entries inside object literals and arrays (http://2ality.com/2017/04/conditional-literal-entries.html):
const arr = [
...(isConditionTrue() ? [{
key: 'value'
}] : [])
];
const obj = {
...(isConditionTrue() ? {key: 'value'} : {})
};
Quite helped me a lot.
Solution 10 - Javascript
This is probably the shortest solution with ES6
console.log({
...true && {foo: 'bar'}
})
// Output: {foo:'bar'}
console.log({
...false && {foo: 'bar'}
})
// Output: {}
Solution 11 - Javascript
I would do this
var a = someCondition ? { b: 5 } : {};
Solution 12 - Javascript
You can add all your undefined values with no condition and then use JSON.stringify to remove them all :
const person = {
name: undefined,
age: 22,
height: null
}
const cleaned = JSON.parse(JSON.stringify(person));
// Contents of cleaned:
// cleaned = {
// age: 22,
// height: null
// }
Solution 13 - Javascript
If the goal is to have the object appear self-contained and be within one set of braces, you could try this:
var a = new function () {
if (conditionB)
this.b = 5;
if (conditionC)
this.c = 5;
if (conditionD)
this.d = 5;
};
Solution 14 - Javascript
This has long been answered, but looking at other ideas I came up with some interesting derivative:
Assign undefined values to the same property and delete it afterwards
Create your object using an anonymous constructor and always assign undefined members to the same dummy member which you remove at the very end. This will give you a single line (not too complex I hope) per member + 1 additional line at the end.
var a = new function() {
this.AlwaysPresent = 1;
this[conditionA ? "a" : "undef"] = valueA;
this[conditionB ? "b" : "undef"] = valueB;
this[conditionC ? "c" : "undef"] = valueC;
this[conditionD ? "d" : "undef"] = valueD;
...
delete this.undef;
};
Solution 15 - Javascript
If you wish to do this server side (without jquery), you can use lodash 4.3.0:
a = _.pickBy({ b: (someCondition? 5 : undefined) }, _.negate(_.isUndefined));
And this works using lodash 3.10.1
a = _.pick({ b: (someCondition? 5 : undefined) }, _.negate(_.isUndefined));
Solution 16 - Javascript
var a = {
...(condition ? {b: 1} : '') // if condition is true 'b' will be added.
}
I hope this is the much efficient way to add an entry based on the condition. For more info on how to conditionally add entries inside an object literals.
Solution 17 - Javascript
Using lodash library, you can use _.omitBy
var a = _.omitBy({
b: conditionB ? 4 : undefined,
c: conditionC ? 5 : undefined,
}, _.IsUndefined)
This results handy when you have requests that are optional
var a = _.omitBy({
b: req.body.optionalA, //if undefined, will be removed
c: req.body.optionalB,
}, _.IsUndefined)
Solution 18 - Javascript
This is the most succinct solution I can come up with:
var a = {};
conditionB && a.b = 5;
conditionC && a.c = 5;
conditionD && a.d = 5;
// ...
Solution 19 - Javascript
i prefere, using code this it, you can run this code
const three = {
three: 3
}
// you can active this code, if you use object `three is null`
//const three = {}
const number = {
one: 1,
two: 2,
...(!!three && three),
four: 4
}
console.log(number);
Solution 20 - Javascript
I think your first approach to adding members conditionally is perfectly fine. I don't really agree with not wanting to have a member b of a with a value of undefined. It's simple enough to add an undefined check with usage of a for loop with the in operator. But anyways, you could easily write a function to filter out undefined members.
var filterUndefined = function(obj) {
var ret = {};
for (var key in obj) {
var value = obj[key];
if (obj.hasOwnProperty(key) && value !== undefined) {
ret[key] = value;
}
}
return ret;
};
var a = filterUndefined({
b: (conditionB? 5 : undefined),
c: (conditionC? 5 : undefined),
d: (conditionD? 5 : undefined),
e: (conditionE? 5 : undefined),
f: (conditionF? 5 : undefined),
g: (conditionG? 5 : undefined),
});
You could also use the delete operator to edit the object in place.
Solution 21 - Javascript
I made a small benchmark with one other option. I like to remove "dead weight" from some objects. Usually falsy values.
Here are the benny results:
clean
const clean = o => {
for (const prop in o) if (!o) delete o[prop];
}
clean({ value });
spread
let a = {
...(value && {b: value})
};
if
let a = {};
if (value) {
a.b = value;
}
results
clean : 84 918 483 ops/s, ±1.16% | 51.58% slower
spread : 20 188 291 ops/s, ±0.92% | slowest, 88.49% slower
if : 175 368 197 ops/s, ±0.50% | fastest
Solution 22 - Javascript
Using lodash library, you can use _.merge
var a = _.merge({}, {
b: conditionB ? 4 : undefined,
c: conditionC ? 5 : undefined,
})
- If conditionB is
false& conditionC istrue, thena = { c: 5 } - If both conditionB & conditionC are
true, thena = { b: 4, c: 5 } - If both conditionB & conditionC are
false, thena = {}
Solution 23 - Javascript
Wrap into an object
Something like this is a bit cleaner
const obj = {
X: 'dataX',
Y: 'dataY',
//...
}
const list = {
A: true && 'dataA',
B: false && 'dataB',
C: 'A' != 'B' && 'dataC',
D: 2000 < 100 && 'dataD',
// E: conditionE && 'dataE',
// F: conditionF && 'dataF',
//...
}
Object.keys(list).map(prop => list[prop] ? obj[prop] = list[prop] : null)
Wrap into an array
Or if you want to use Jamie Hill's method and have a very long list of conditions then you must write ... syntax multiple times. To make it a bit cleaner, you can just wrap them into an array, then use reduce() to return them as a single object.
const obj = {
X: 'dataX',
Y: 'dataY',
//...
...[
true && { A: 'dataA'},
false && { B: 'dataB'},
'A' != 'B' && { C: 'dataC'},
2000 < 100 && { D: 'dataD'},
// conditionE && { E: 'dataE'},
// conditionF && { F: 'dataF'},
//...
].reduce(( v1, v2 ) => ({ ...v1, ...v2 }))
}
Or using map() function
const obj = {
X: 'dataX',
Y: 'dataY',
//...
}
const array = [
true && { A: 'dataA'},
false && { B: 'dataB'},
'A' != 'B' && { C: 'dataC'},
2000 < 100 && { D: 'dataD'},
// conditionE && { E: 'dataE'},
// conditionF && { F: 'dataF'},
//...
].map(val => Object.assign(obj, val))
Solution 24 - Javascript
Define a var by let and just assign new property
let msg = {
to: "[email protected]",
from: "[email protected]",
subject: "Contact form",
};
if (file_uploaded_in_form) { // the condition goes here
msg.attachments = [ // here 'attachments' is the new property added to msg Javascript object
{
content: "attachment",
filename: "filename",
type: "mime_type",
disposition: "attachment",
},
];
}
Now the msg become
{
to: "[email protected]",
from: "[email protected]",
subject: "Contact form",
attachments: [
{
content: "attachment",
filename: "filename",
type: "mime_type",
disposition: "attachment",
},
]
}
In my opinion this is very simple and easy solution.
Solution 25 - Javascript
For the sake of completeness you can use Object.defineProperty() if you want to add additional descriptors. Note I purposely added enumerable: true otherwise the property wouldn't appear in the console.log(). The advantage with this approach is that you can also use Object.defineProperties() if you want to add multiple new properties (However, in this way every property will be dependent on the same condition...)
const select = document.getElementById("condition");
const output = document.getElementById("output");
let a = {};
let b = {};
select.onchange = (e) => {
const condition = e.target.value === "true";
condition
? Object.defineProperty(a, "b", {
value: 5,
enumerable: true,
})
: (a = {});
condition
? Object.defineProperties(b, {
c: {
value: 5,
enumerable: true,
},
d: {
value: 6,
enumerable: true,
},
e: {
value: 7,
enumerable: true,
},
})
: (b = {});
outputSingle.innerText = JSON.stringify(a);
outputMultiple.innerText = JSON.stringify(b);
};
Condition:
<select id="condition">
<option value="false">false</option>
<option value="true">true</option>
</select>
<br/>
<br/>
Single Property: <pre id="outputSingle">{}</pre><br/>
Multiple Properties: <pre id="outputMultiple">{}</pre>