Typescript - How the class instance is created here please?

Question

I have some code here. My problem is that I don't manage to understand the way the instance is created. Why we use " : Calculator" ( line 3 ) to use after " new Calculator" (line 4). There is a difference ? I know that I have a problem concerning Typescript but I found this code piece while I was looking for testing in Angular. Also I searched into some tutorials but I don't find explanation.

import { Calculator } from './calculator';

describe('Calculator', () => {

    let calculator: Calculator;

    beforeEach(() => {
        calculator = new Calculator();
    });

});

Typescript is a strongly-typed language. let calculator: Calculator is specifying that the variable calculator is of the Calculator type. — developing2020
– developing2020, Commented May 21, 2019 at 14:50
It's unclear what your question is: let calculator: Calculator declares a variable of type Calculator once, but doesn't assign it. calculator = new Calculator(); is called before every test, assigning a new Calculator for each test. Because of how jasmine, works, you often do calculator = null in an afterEach to avoid memory problems; — Ruan Mendes
– Ruan Mendes, Commented May 21, 2019 at 14:52

Jalu · Accepted Answer · 2019-05-21 14:54:20Z

1

In short, you use the : Calculator to give a type to that variable, on the other hand it is declared outside beforeEach's scope so it can be accesible on the tests.

If you did

beforeEach(() => {
    let calculator = new Calculator();
});

calculator won't be accessible.

answered May 21, 2019 at 14:54

Jalu

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1 Comment

Jalu Over a year ago

@N.T let is recommended you can use var, let is used to 'scope' the variable. Creating the instance inside beforeEach will make the calculator variable unusable.

Ashish Ranjan · Accepted Answer · 2019-05-21 14:51:40Z

1

Your "calculator.ts" file probably has something like:

export class Calculator {
    ...
}

When you do import { Calculator } from './calculator';, you import the Calculator class in your current file.

let calculator: Calculator; will declare a variable calculator giving it a type Calculator <- So you specifically say that you will have Calculator objects in this variable. This is for Typescripts understanding and code completion,

This will create a new instance of the class.

 calculator = new Calculator();

answered May 21, 2019 at 14:51

Ashish Ranjan

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Vitalii Chmovzh · Accepted Answer · 2019-05-21 14:55:20Z

1

import { Calculator } from './calculator'; // importing Calculator class

describe('Calculator', () => { // Describing feature in BDD manner

    let calculator: Calculator; 
    // Declaring variable calculator so it's accessible within whole describe() block

    beforeEach(() => { // This hook will be called before each test in your feature
        calculator = new Calculator(); 
        // and therefore will create new instance of a Calculator for each test
    });

    // Here you probably will see something like

    it('should return sum of 2 numbers', () => {
        const result = calculator.add(2,3); // actual instance used
        expect(result).toEqual(5);
    });

});

answered May 21, 2019 at 14:55

Vitalii Chmovzh

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Miguel Cabrera · Accepted Answer · 2019-05-21 14:57:54Z

1

The line 3, means the type declaration of your variable, but at first is undefined, so you need to create a new instance of your type, that's why in the next line it does new Calculator().

answered May 21, 2019 at 14:57

Miguel Cabrera

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