How to unit test a filter in AngularJS 1.x

Question

How do you unit test a filter in Angular?

Answer 1

Here you have a runnable filter test example.

angular.module('myModule', []).filter('multiplier', function() {
  return function(number, multiplier) {
    if (!angular.isNumber(number)) {
      throw new Error(number + " is not a number!");
    }
    if (!multiplier) {
      multiplier = 2;
    }
    return number * multiplier;
  }
});

describe('multiplierFilter', function() {
  var filter;

  beforeEach(function() {
    module('myModule');
    inject(function(multiplierFilter) {
      filter = multiplierFilter;
    });
  });

  it('multiply by 2 by default', function() {
    expect(filter(2)).toBe(4);
    expect(filter(3)).toBe(6);
  });

  it('allow to specify custom multiplier', function() {
    expect(filter(2, 4)).toBe(8);
  });

  it('throws error on invalid input', function() {
    expect(function() {
      filter(null);
    }).toThrow();
  });
});

Note: this answer was created after the SO Documentation Sunset based on the AngularJS unit test filter example and a suggestion on Meta that all valuable documentation content should be converted into answers.