Math functions in AngularJS bindings

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Is there a way to use math functions in AngularJS bindings?

e.g.

The percentage is {{Math.round(100*count/total)}}%

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一整个雨季

2020-11-29 18:28
This is more or less a summary of three answers (by Sara Inés Calderón, klaxon and Gothburz), but as they all added something important, I consider it worth joining the solutions and adding some more explanation.

Considering your example, you can do calculations in your template using:
```
{{ 100 * (count/total) }}
```
However, this may result in a whole lot of decimal places, so using filters is a good way to go:
```
{{ 100 * (count/total) | number }}
```
By default, the number filter will leave up to three fractional digits, this is where the fractionSize argument comes in quite handy ({{ 100 * (count/total) | number:fractionSize }}), which in your case would be:
```
{{ 100 * (count/total) | number:0 }}
```
This will also round the result already:
```
angular.module('numberFilterExample', [])
  .controller('ExampleController', ['$scope',
    function($scope) {
      $scope.val = 1234.56789;
    }
  ]);
```
```
<!doctype html>
<html lang="en">
  <head>  
    <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>
  </head>
  <body ng-app="numberFilterExample">
    <table ng-controller="ExampleController">
      <tr>
        <td>No formatting:</td>
        <td>
          <span>{{ val }}</span>
        </td>
      </tr>
      <tr>
        <td>3 Decimal places:</td>
        <td>
          <span>{{ val | number }}</span> (default)
        </td>
      </tr>
      <tr>
        <td>2 Decimal places:</td>
        <td><span>{{ val | number:2 }}</span></td>
      </tr>
      <tr>
        <td>No fractions: </td>
        <td><span>{{ val | number:0 }}</span> (rounded)</td>
      </tr>
    </table>
  </body>
</html>
```
Last thing to mention, if you rely on an external data source, it probably is good practise to provide a proper fallback value (otherwise you may see NaN or nothing on your site):
```
{{ (100 * (count/total) | number:0) || 0 }}
```
Sidenote: Depending on your specifications, you may even be able to be more precise with your fallbacks/define fallbacks on lower levels already (e.g. {{(100 * (count || 10)/ (total || 100) | number:2)}}). Though, this may not not always make sense..
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名媛妹妹

2020-11-29 18:32

This is a hairy one to answer, because you didn't give the full context of what you're doing. The accepted answer will work, but in some cases will cause poor performance. That, and it's going to be harder to test.

If you're doing this as part of a static form, fine. The accepted answer will work, even if it isn't easy to test, and it's hinky.

If you want to be "Angular" about this:

You'll want to keep any "business logic" (i.e. logic that alters data to be displayed) out of your views. This is so you can unit test your logic, and so you don't end up tightly coupling your controller and your view. Theoretically, you should be able to point your controller at another view and use the same values from the scopes. (if that makes sense).

You'll also want to consider that any function calls inside of a binding (such as {{}} or ng-bind or ng-bind-html) will have to be evaluated on every digest, because angular has no way of knowing if the value has changed or not like it would with a property on the scope.

The "angular" way to do this would be to cache the value in a property on the scope on change using an ng-change event or even a $watch.

For example with a static form:

angular.controller('MainCtrl', function($scope, $window) {
   $scope.count = 0;
   $scope.total = 1;

   $scope.updatePercentage = function () {
      $scope.percentage = $window.Math.round((100 * $scope.count) / $scope.total);
   };
});

<form name="calcForm">
   <label>Count <input name="count" ng-model="count" 
                  ng-change="updatePercentage()"
                  type="number" min="0" required/></label><br/>
   <label>Total <input name="total" ng-model="total"
                  ng-change="updatePercentage()"
                  type="number" min="1" required/></label><br/>
   <hr/>
   Percentage: {{percentage}}
</form>

And now you can test it!

describe('Testing percentage controller', function() {
  var $scope = null;
  var ctrl = null;

  //you need to indicate your module in a test
  beforeEach(module('plunker'));

  beforeEach(inject(function($rootScope, $controller) {
    $scope = $rootScope.$new();

    ctrl = $controller('MainCtrl', {
      $scope: $scope
    });
  }));

  it('should calculate percentages properly', function() {
    $scope.count = 1;
    $scope.total = 1;
    $scope.updatePercentage();
    expect($scope.percentage).toEqual(100);

    $scope.count = 1;
    $scope.total = 2;
    $scope.updatePercentage();
    expect($scope.percentage).toEqual(50);

    $scope.count = 497;
    $scope.total = 10000;
    $scope.updatePercentage();
    expect($scope.percentage).toEqual(5); //4.97% rounded up.

    $scope.count = 231;
    $scope.total = 10000;
    $scope.updatePercentage();
    expect($scope.percentage).toEqual(2); //2.31% rounded down.
  });
});

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青春惊慌失措

2020-11-29 18:34

If you're looking to do a simple round in Angular you can easily set the filter inside your expression. For example:

{{ val | number:0 }}

See this CodePen example & for other number filter options.

Angular Docs on using Number Filters

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谎友^

2020-11-29 18:37
Either bind the global Math object onto the scope (remember to use $window not window)
```
$scope.abs = $window.Math.abs;
```
Use the binding in your HTML:
```
<p>Distance from zero: {{abs(distance)}}</p>
```
Or create a filter for the specific Math function you're after:
```
module.filter('abs', ['$window', function($window) {
  return function(n) {
    return $window.Math.abs($window.parseInt(n));
  };
});
```
Use the filter in your HTML:
```
<p>Distance from zero: {{distance | abs}}</p>
```
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我在风中等你

2020-11-29 18:38
You have to inject Math into your scope, if you need to use it as $scope know nothing about Math.

Simplest way, you can do
```
$scope.Math = window.Math;
```
in your controller. Angular way to do this correctly would be create a Math service, I guess.
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野趣味

2020-11-29 18:45

Why not wrap the whole math obj in a filter?

var app = angular.module('fMathFilters',[]);


function math() {
    return function(input,arg) {
        if(input) {
            return Math[arg](input);
        }

        return 0;
    }
}

return app.filter('math',[math]);

and to use:

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