Say I have an array, like so:
arr = [{}, {}, {}, {}]
I want to be able to know the length of the array, only counting objects within in it that have at least one property.
[{}, {name: "Derby County", odds: 2}, {}, {}] // 1
[{}, {name: "Derby County", odds: 2}, {name: "Fullham", odds: 3} ,{}] // 2
How do I achieve this?
arr.filter(x => Object.keys(x).length).length
As explained in other answers, Object.keys() returns property names from a given object. The inner .length is a shortcut to filter only items that have at least one property. The outer .length tells how many objects fit the description.
UPDATE:
The [].filter() method takes a function that returns a thruthy/falsy value. A number greater than 0 is thruthy, so it's the same as .length !== 0.
The assumption here is that any element contained in the array is non-null. Under this assumption it makes no sense checking the object for null inside the [].filter(). When using TypeScript, it's a static check for arr. If the assumption is broken, then an error is thrown, which it's something I usually desire. I don't hide runtime errors. If there's a runtime error here, I'll review the assumption. Yet I'm not sure it's the case here.
arr.filter(x => x ? Object.keys(x).length > 0 : false).length;
.length access made you think it would accumulate properties?If you do:
arr.map(x=> Object.keys(x).length)
you'll get:
[ 0, 0, 0, 1 ]
If the object is empty then there are no keys and so its length is 0.
If you need a true/false result do:
arr.map(x=> Object.keys(x).length).some(x=>x>0)
console.log("[{},{}]", [{},{}].map(x=> Object.keys(x).length).some(x=>x>0))
console.log("[{},{a: 1}]", [{},{a: 1}].map(x=> Object.keys(x).length).some(x=>x>0))
Object.keys(obj).lengthwill yield a non-zero result when there's at least one iterable, non-inherited property.detect when something is addedthis has more than one way to interpret it, with quite different solutions