TIL; typescript - classes | public fields in constructor

Seyram Ofori — Thu, 15 Dec 2022 14:57:12 +0000

the use of public on arguments on the constructor is a shorthand that allows us to automatically create properties with that name

class Student {
    fullName: String;

    constructor(
        public firstName: string,
        public lastName: string
    ){
        this.fullName = firstName + " " + lastName;
    }

}

in this case, the class has been given 3 public properties; fullName, firstName and lastName. firstName and lastName were created as public properties on the class by using the public keyword in the constructor params

classes in TS are just a shorthand for the same prototype-based OO that is frequently used in JS

TIL; JavaScript Jest - matchers

Seyram Ofori — Thu, 15 Dec 2022 06:38:08 +0000

matchers

matchers allow you to test values by comparing the actual value to the expected value
expect returns an expectation object. you call matchers on this object to achieve the comparison of actual to expected.
toBe matcher uses [Object.is] to test exact equality.
if you want to check the value of an object, use toEqual or toStrictEqual instead.
- what does ‘check the value of an object’ mean?
toBe vs toEqual vs toStrictEqual. when to use which?
- prefer toStrictEqual over toEqual, cos toEqual simply ignores undefined values
- under which conditions would you want to ignore undefined values?
you can easily test the opposite of a matcher using .not. eg: expect(a+b).not.toStrictEqual(0)

truthiness

matchers that allow you to be specific about the kind of truthy value you want
there’s 5 of them
toBeNull matches only null
toBeUndefined matches only undefined
toBeDefined is the opposite of toBeUndefined
toBeTruthy matches anything that an if statement treats as true
toBeFalsy matches anything that an if statement treats as false
you should use the matcher that most precisely corresponds to what you want your code to be doing

numbers

there’s 7 matchers that can be used for numbers
toBeGreaterThan
toBeGreatherThanOrEqual
toBeLessThan
toBeLessThanOrEqual
toBe
toEqual

toBeCloseTo ; to be used for floating point equality. cos we don’t want a test to depend on a tiny rounding error.

const value = 0.1 + 0.2;
expect(value).toBe(0.3); // this won't work cos of rounding error
expect(value).toBeCloseTo(0.3); // this works

toBe and toEqual are equivalent for numbers

strings

you can check strings against regular expressions with toMatch
- expect('Christoph').toMatch(/stop/)

arrays and iterables

you can check if an array or iterable contains a particular item using toContain
- expect(['diapers', 'paper towels']).toContain('paper towels');
JS iterables include the ff
- string
- array
- TypedArray
- Map
- Set
- Segments (returned by Intl, Segmenter, prototype, segment())

an iterable is defined by the fact that its prototype object implements an @@iterator method

exceptions

to test whether a particular function throws an error when it’s called, use toThrow
to check the error message, you can pass a regex to the toThrow() matcher
the function that throws an exception needs to be invoked within a wrapping function otherwise the toThrow() assertion will fail.

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