如何计算AngularJS应用列表中特定项目的数量

Question

I am using the following code to try and generate a list showing the count of each item in the letters list within an AngularJS controller:

$scope.letters = ['a','b','c'];
$scope.letterdata = ['a','b','a','c','b','a'];

$scope.counts = $scope.letters.map(function (x) {
    $scope.letterdata.filter(function (y) {
        return (x == y);
    }).length;
});

I would like to get [3,2,1] as the output, but this is showing as [null,null,null] .

Answer 1

You're missing return statement inside map() .

$scope.counts = $scope.letters.map(function (x) {
    return $scope.letterdata.filter(function (y) {
    ^^^^^^
        return (x == y);
    }).length;
});

 var letters = ['a', 'b', 'c']; var letterdata = ['a', 'b', 'a', 'c', 'b', 'a']; var counts = letters.map(function (x) { return letterdata.filter(function (y) { return(x == y); }).length; }); console.log(counts); 

but this is showing as [null,null,null] .

You should be getting [ undefined, undefined, undefined ] as nothing is returned, by default undefined is returned.

---

Equivalent code written using ES2015 arrow function .

$scope.letters.map(x => $scope.letterdata.filter(y => x == y).length)

 var letters = ['a', 'b', 'c']; var letterdata = ['a', 'b', 'a', 'c', 'b', 'a']; var counts = letters.map(x => letterdata.filter(y => x == y).length); console.log(counts); 

Answer 2

var list = ['a', 'b', 'c'];
var data = ['a', 'b', 'a', 'c', 'b', 'a'];

_.reduce(list, function(counts, value) {
    counts.push(_.size(data) - _.size(_.without(data, value)));
    return counts;
}, []);