How do I test if something is an array of a given class with Jasmine?

Question

I am writing an Angular application. I have a test for my component (losely following this manual), where I want to check that my ngOnInit() calls the provided (mock) service, and initialises my property foos with an array of Foo objects.

it('should have foos after Angular calls ngOnInit', () => {
  component.ngOnInit();
  expect(component.foos).toBeInstanceOf(Foo);
});

The above works for a single object, but not for an array. If I check for Array the test passes, but isn't particularly useful.

expect(component.foos).toBeInstanceOf(Array);

I tried Foo[], but that gave me an error.

An element access expression should take an argument.

I realise this test is not useful in general because typescript will complain if it returns something that is he wrong class. But I still want to know as a matter of principle.

user_3689450 · Answer

There is no way to do this check in Jasmine out of the box, to my knowledge. However, you can use some additional code to achieve this result.

You can check the contents yourself with Array#every() and see if they match. Then use the toBeTrue() matcher.
```
expect(someArray.every(x => x instanceof Foo))
  .toBeTrue("Some items don't match");
```
Loop over the array and use the toBeInstanceOf() matcher against each item. You can also add withContext() to give additional information, otherwise you won't know which item failed the matcher:
```
for (const [index, x] of someArray.entries()) {
  expect(x)
    .withContext(`index [${index}]`)
    .toBeInstanceOf(Foo, "hello");
}
```

Loop over the array and manually call fail() with an error message:

for (const [index, x] of someArray.entries()) {
  if (!(x instanceof Foo)) 
    fail(`index [${index}] is not Foo`, x);
}

class Foo { constructor(num) { this._num = num} }; // some dummy implementation
class Bar { constructor(str) { this._srr = str} }; // some dummy implementation

const arrayOfFoos  = [new Foo(1), new Foo(2), new Foo(3)];
const arrayOfBars  = [new Bar("one"), new Bar("two"), new Bar("three")];
const arrayOfMixed = [new Foo(1), new Bar("two"), new Foo(3)];

describe("All Foos", function() {
  beforeEach(function() {
    this.array = arrayOfFoos;
  });
  
  it("every() + toBeTrue()", function() {
    expect(this.array.every(x => x instanceof Foo))
      .toBeTrue("Some items don't match");
  });
  
  it("loop + toBeInstanceOf()", function() {
    for (const [index, x] of this.array.entries()) {
      expect(x)
        .withContext(`index [${index}]`)
        .toBeInstanceOf(Foo);
    }
  });
  
  it("loop + fail()", function() {
    for (const [index, x] of this.array.entries()) {
      if (!(x instanceof Foo)) 
        fail(`index [${index}] is not Foo`, x);
    }
  });
});

describe("All Bars", function() {
  beforeEach(function() {
    this.array = arrayOfBars;
  });
  
  it("every() + toBeTrue()", function() {
    expect(this.array.every(x => x instanceof Foo))
      .toBeTrue("Some items don't match");
  });
  
  it("loop + toBeInstanceOf()", function() {
    for (const [index, x] of this.array.entries()) {
      expect(x)
        .withContext(`index [${index}]`)
        .toBeInstanceOf(Foo);
    }
  });
  
  it("loop + fail()", function() {
    for (const [index, x] of this.array.entries()) {
      if (!(x instanceof Foo)) 
        fail(`index [${index}] is not Foo`, x);
    }
  });
});

describe("Mixed", function() {
  beforeEach(function() {
    this.array = arrayOfMixed;
  });
  
  it("every() + toBeTrue()", function() {
    expect(this.array.every(x => x instanceof Foo))
      .toBeTrue("Some items don't match");
  });
  
  it("loop + toBeInstanceOf()", function() {
    for (const [index, x] of this.array.entries()) {
      expect(x)
        .withContext(`index [${index}]`)
        .toBeInstanceOf(Foo);
    }
  });
  
  it("loop + fail()", function() {
    for (const [index, x] of this.array.entries()) {
      if (!(x instanceof Foo)) 
        fail(`index [${index}] is not Foo`, x);
    }
  });
});

How do I test if something is an array of a given class with Jasmine?

1 Answers