Best way to count the number of objects in a array with different object property p values.
function([{"p":"a"},{"p":"b"},{"p":"a"}]){
// some code
}
// in this case return 2
5 Answers
You can use Array.prototype.filter() to keep values that you want. In this case, you must create a variable temp for storing duplicate values and also within the filter function returns true if it does not exist in case. So you have a new array of unique values.
var arr = [{"p":"a"},{"p":"b"},{"p":"a"}], temp = [];
var arrUniques = arr.filter(function(obj){
return temp.indexOf(obj.p) === -1 ? !!temp.push(obj.p) : false
});
alert(arrUniques.length)
answered Jul 2, 2015 at 6:05
Walter Chapilliquen - wZVanG
12.1k8 gold badges44 silver badges64 bronze badges
Comments
With a Map:
var props = new Map();
for (var i = 0; i < array.length; i++) {
var prop = array[i].p,
count = props.get(prop) || 0;
props.set(prop, count + 1);
}
var size = props.size;
If your properties can be safely casted to strings, you can use a common object:
var props = {};
...
var size = Object.keys(props).length;
Otherwise, Map is your answer.
Comments
function getUniquePropertyCount(a, p) {
return a.reduce(function (res, el) {
!~res.indexOf(el[p]) && res.push(el[p]);
return res;
}, []).length;
}
document.write(getUniquePropertyCount([{ "p": "a" }, { "p": "b" }, { "p": "a" }], 'p'));
Comments
I suppose this is one of those questions where if you ask four programmers, you'll get five answers.
The other answers so far show some interesting approaches. I would watch out for browser compatibility; for example Map() is only available in the newest browsers, not in IE 10 or prior.
So here's one more solution. It's a bit more code, but it's pretty easy to understand, and it works in every browser:
function countUniqueProperties( key, array ) {
var count = 0, values = {};
for( var i = 0; i < array.length; ++i ) {
var value = array[i][key];
if( ! Object.prototype.hasOwnProperty.call( values, value) ) {
++count;
values[value] = true;
}
}
return count;
}
countUniqueProperties( 'p', [ {p:'a'}, {p:'b'}, {p:'a'} ] );
The one complicated part here is the Object.prototype.hasOwnProperty.call(values,value). You could just use values.hasOwnProperty(value), but that would fail if one of your property values was the string "hasOwnProperty":
countUniqueProperties( 'p', [ {p:'a'}, {p:'hasOwnProperty'}, {p:'a'} ] );
Using the longer form avoids this issue.
.map().filter().lengthorreduce()<= (disclaimer, my blog post)