What is the best way to verify if an object is an instance of a particular class using Jasmine testing?

Question

What is the best way to verify if an object is an instance of a particular class using Jasmine testing?

In the process of developing an Angular application, I am currently working on testing a component using this guide. My goal is to ensure that when my ngOnInit() method is called, it correctly initializes my foos property with an array of Foo objects based on a provided (mock) service.

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

While the test works for a single object, it doesn't fulfill the requirement for an array. Testing against Array passes but lacks meaningful validation.

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

Attempting to use Foo[] resulted in an error message.

An element access expression should take an argument.

Although TypeScript already provides type checking, I still want to confirm this behavior as a matter of principle.

typescript jasmine

Answer 1

Answer №1

There isn't a built-in way to perform this validation in Jasmine, as far as I know. However, you can incorporate additional code to accomplish the desired outcome.

You have the option to manually compare the contents using Array#every() and then utilize the toBeTrue() matcher.
```
expect(someArray.every(x => x instanceof Foo))
  .toBeTrue("Some items don't match");
```
Iterate through the array and apply the toBeInstanceOf() matcher for each element. You can also include withContext() to provide more details in case of a mismatch:
```
for (const [index, x] of someArray.entries()) {
  expect(x)
    .withContext(`index [${index}]`)
    .toBeInstanceOf(Foo, "hello");
}
```

Loop through the array and trigger fail() with an error message when necessary:

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

<link href="https://cdnjs.cloudflare.com/ajax/libs/jasmine/3.6.0/jasmine.min.css" rel="stylesheet" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/jasmine/3.6.0/jasmine.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jasmine/3.6.0/jasmine-html.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jasmine/3.6.0/boot.min.js"></script>

Answer 2

There isn't a built-in way to perform this validation in Jasmine, as far as I know. However, you can incorporate additional code to accomplish the desired outcome.

You have the option to manually compare the contents using Array#every() and then utilize the toBeTrue() matcher.
```
expect(someArray.every(x => x instanceof Foo))
  .toBeTrue("Some items don't match");
```
Iterate through the array and apply the toBeInstanceOf() matcher for each element. You can also include withContext() to provide more details in case of a mismatch:
```
for (const [index, x] of someArray.entries()) {
  expect(x)
    .withContext(`index [${index}]`)
    .toBeInstanceOf(Foo, "hello");
}
```

Loop through the array and trigger fail() with an error message when necessary:

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

<link href="https://cdnjs.cloudflare.com/ajax/libs/jasmine/3.6.0/jasmine.min.css" rel="stylesheet" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/jasmine/3.6.0/jasmine.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jasmine/3.6.0/jasmine-html.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jasmine/3.6.0/boot.min.js"></script>

What is the best way to verify if an object is an instance of a particular class using Jasmine testing?

Answer №1

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