Is it possible to lengthen a function in TypeScript?

Question

Is it possible to lengthen a function in TypeScript?

I am part of a team responsible for maintaining a unique JavaScript library that generates spy functions. These spy functions are designed to allow users to monitor how a function is called, primarily used in the context of unit testing.

Our library creates specialized functions with additional properties that enable users to analyze the calls made to them.

We are currently exploring the possibility of creating an innovative TypeScript definition that would enable us to pass these specialized functions into methods requiring a standard function while also retaining their extra properties.

While the following code snippet is not valid, it showcases our vision:

class Spy extends function {
    wasCalled: () => boolean;
    ...
}

Implementing this concept would empower me to seamlessly integrate a spy into a function with the following signature:

function subjectUnderTest(callback:() => void) {
    ...
}

typescript

Answer 1

Answer №1

A "hybrid type" is referred to in the TypeScript handbook as a combination of a function type and a regular interface.

interface Logger {
    (message: string, level: number): void; // Example
    logCount(): number;
}

var logger: Logger = createLoggerInstance();
logger("Hello", 1);
if (logger.logCount() > 0) {
    // Do something...
}

Answer 2

A "hybrid type" is referred to in the TypeScript handbook as a combination of a function type and a regular interface.

interface Logger {
    (message: string, level: number): void; // Example
    logCount(): number;
}

var logger: Logger = createLoggerInstance();
logger("Hello", 1);
if (logger.logCount() > 0) {
    // Do something...
}

Answer 3

Answer №2

In my quest to enhance a class with a function, I delved into creating a TypeScript-exclusive solution. The uncertainty looms over whether this approach is wise, as ingenious solutions may not always be the best ones. Your mileage may vary.

A shoutout to Mattias Buelens for laying the groundwork with a partial answer - I am building upon it.

// mirroring Mattias' response
interface Spy {
    (foo: string, bar: number): boolean // Just an example
    wasCalled(): boolean
}

// presenting the ultimate fix!
class Spy {
    _wasCalled: boolean
    _baz: boolean // Just an example

    private constructor(baz: boolean) {
        this._wasCalled = false
        this._baz = baz
    }

    wasCalled(): boolean {
        return this._wasCalled
    }

    toString() { return '[object Spy]' }

    static create(baz: boolean) {
        const f = <Spy>function(this: Spy, foo: string, bar: number): boolean {
            // Put your logic here. Utilize f instead of this!
            console.log('wasCalled', f.wasCalled())
            f._wasCalled = true
        }
        const spy = new Spy(baz)
        Object.assign(f, spy)
        Object.setPrototypeOf(f, Spy.prototype)

        return f
    }
}

The concept revolves around crafting a function alongside the Spy instance and then syncing both the prototype and properties to the function. A static method returns the instance. An added perk is the inclusion of the toString() method.

const spy = Spy.create(true)
console.log('calling spy', spy('foo', 42))
console.log('instanceof', spy instanceof Spy)

And voilà! It performs seamlessly.

I harbor doubts about the viability of new Spy() since we must assign to a function rather than vice versa. Moreover, due to our inability to substitute this, transforming it into a callable entity remains unattainable. Envisioning a scenario where a class extends a functional constructor akin to

class Spy2 extends function() {} {}

seems promising, yet operationalizing it poses a challenge.

Answer 4

In my quest to enhance a class with a function, I delved into creating a TypeScript-exclusive solution. The uncertainty looms over whether this approach is wise, as ingenious solutions may not always be the best ones. Your mileage may vary.

A shoutout to Mattias Buelens for laying the groundwork with a partial answer - I am building upon it.

// mirroring Mattias' response
interface Spy {
    (foo: string, bar: number): boolean // Just an example
    wasCalled(): boolean
}

// presenting the ultimate fix!
class Spy {
    _wasCalled: boolean
    _baz: boolean // Just an example

    private constructor(baz: boolean) {
        this._wasCalled = false
        this._baz = baz
    }

    wasCalled(): boolean {
        return this._wasCalled
    }

    toString() { return '[object Spy]' }

    static create(baz: boolean) {
        const f = <Spy>function(this: Spy, foo: string, bar: number): boolean {
            // Put your logic here. Utilize f instead of this!
            console.log('wasCalled', f.wasCalled())
            f._wasCalled = true
        }
        const spy = new Spy(baz)
        Object.assign(f, spy)
        Object.setPrototypeOf(f, Spy.prototype)

        return f
    }
}

The concept revolves around crafting a function alongside the Spy instance and then syncing both the prototype and properties to the function. A static method returns the instance. An added perk is the inclusion of the toString() method.

const spy = Spy.create(true)
console.log('calling spy', spy('foo', 42))
console.log('instanceof', spy instanceof Spy)

And voilà! It performs seamlessly.

I harbor doubts about the viability of new Spy() since we must assign to a function rather than vice versa. Moreover, due to our inability to substitute this, transforming it into a callable entity remains unattainable. Envisioning a scenario where a class extends a functional constructor akin to

class Spy2 extends function() {} {}

seems promising, yet operationalizing it poses a challenge.

Is it possible to lengthen a function in TypeScript?

Answer №1

Answer №2

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