Get functions (methods) of a class

Question

I have to dynamically fetch the properties and functions of a ES6 class. Is this even possible?

Using a for...in loop, I only get to loop through the properties of a class instance:

class Foo {
  constructor() {
    this.bar = "hi";
  }
  someFunc() {
    console.log(this.bar);
  }
}
var foo = new Foo();
for (var idx in foo) {
  console.log(idx);
}

Output:

bar

Solution 1

This function will get all functions. Inherited or not, enumerable or not. All functions are included.

function getAllFuncs(toCheck) {
    const props = [];
    let obj = toCheck;
    do {
        props.push(...Object.getOwnPropertyNames(obj));
    } while (obj = Object.getPrototypeOf(obj));
    
    return props.sort().filter((e, i, arr) => { 
       if (e!=arr[i+1] && typeof toCheck[e] == 'function') return true;
    });
}

Do test

getAllFuncs([1,3]);

console output:

["constructor", "toString", "toLocaleString", "join", "pop", "push", "concat", "reverse", "shift", "unshift", "slice", "splice", "sort", "filter", "forEach", "some", "every", "map", "indexOf", "lastIndexOf", "reduce", "reduceRight", "entries", "keys", "constructor", "toString", "toLocaleString", "valueOf", "hasOwnProperty", "isPrototypeOf", "propertyIsEnumerable", "__defineGetter__", "__lookupGetter__", "__defineSetter__", "__lookupSetter__"]

Note

It doesn't return functions defined via symbols;

Solution 2

The members of a class are not enumerable. To get them, you have to use Object.getOwnPropertyNames:

var propertyNames = Object.getOwnPropertyNames(Object.getPrototypeOf(foo));
// or
var propertyNames = Object.getOwnPropertyNames(Foo.prototype);

Of course this won't get inherited methods. There is no method that can give you all of them. You'd have to traverse the prototype chain and get the properties for each prototype individually.

Solution 3

ES6 adds Reflection which makes the code to do this a bit cleaner.

function getAllMethodNames(obj) {
  let methods = new Set();
  while (obj = Reflect.getPrototypeOf(obj)) {
    let keys = Reflect.ownKeys(obj)
    keys.forEach((k) => methods.add(k));
  }
  return methods;
}


/// a simple class hierarchy to test getAllMethodNames


// kind of like an abstract base class
class Shape {
  constructor() {}
  area() {
    throw new Error("can't define area for generic shape, use a subclass")
  }
}

// Square: a shape with a sideLength property, an area function and getSideLength function
class Square extends Shape {
  constructor(sideLength) {
    super();
    this.sideLength = sideLength;
  }
  area() {
    return this.sideLength * this.sideLength
  };
  getSideLength() {
    return this.sideLength
  };
}

// ColoredSquare: a square with a color
class ColoredSquare extends Square {
  constructor(sideLength, color) {
    super(sideLength);
    this.color = color;
  }
  getColor() {
    return this.color
  }
}


let temp = new ColoredSquare(2, "red");
let methods = getAllMethodNames(temp);
console.log([...methods]);

Solution 4

There were a few issues in @MuhammadUmer answer for me (symbols, index i+1, listing of Object methods, etc...) so taking inspiration from it, I came up with this

(warning Typescript compiled to ES6)

const getAllMethods = (obj) => {
    let props = []

    do {
        const l = Object.getOwnPropertyNames(obj)
            .concat(Object.getOwnPropertySymbols(obj).map(s => s.toString()))
            .sort()
            .filter((p, i, arr) =>
                typeof obj[p] === 'function' &&  //only the methods
                p !== 'constructor' &&           //not the constructor
                (i == 0 || p !== arr[i - 1]) &&  //not overriding in this prototype
                props.indexOf(p) === -1          //not overridden in a child
            )
        props = props.concat(l)
    }
    while (
        (obj = Object.getPrototypeOf(obj)) &&   //walk-up the prototype chain
        Object.getPrototypeOf(obj)              //not the the Object prototype methods (hasOwnProperty, etc...)
    )

    return props
}

This function will list all methods of an instance of the class including those inherited, but not the constructor and those of the Object prototype.

Test

The function returns

[ 'asyncMethod',
  'echo',
  'generatorMethod',
  'ping',
  'pong',
  'anotherEcho' ]

listing the methods of an instance of TestClass (typescript)

class Echo  {
    echo(data: string): string {
        return data
    }
    anotherEcho(data: string): string {
        return `Echo ${data}`
    }
}


class TestClass extends Echo {

    ping(data: string): string {
        if (data === 'ping') {
            return 'pong'
        }
        throw new Error('"ping" was expected !')
    }

    pong(data: string): string {
        if (data === 'pong') {
            return 'ping'
        }
        throw new Error('"pong" was expected !')
    }

    //overridden echo
    echo(data: string): string {
        return 'blah'
    }

    async asyncMethod(): Promise<string> {
        return new Promise<string>((resolve: (value?: string) => void, reject: (reason?: any) => void) => {
            resolve('blah')
        })
    }

    * generatorMethod(): IterableIterator<string> {
        yield 'blah'
    }
}

Solution 5

To make members of class enumerable you can use Symbol.iterator

I had to get all allowed methods of object (including inherited). So i created class "Enumerable" and all my base classes inherited from him.

class Enumerable {
  constructor() {

    // Add this for enumerate ES6 class-methods
    var obj = this;

    var getProps = function* (object) {
      if (object !== Object.prototype) {
        for (let name of Object.getOwnPropertyNames(object)) {
          let method = object[name];
          // Supposedly you'd like to skip constructor and private methods (start with _ )
          if (method instanceof Function && name !== 'constructor' && name[0] !== '_')
            yield name;
        }
        yield* getProps(Object.getPrototypeOf(object));
      }
    }

    this[Symbol.iterator] = function*() {
      yield* getProps(obj);
    }
    // --------------
  }
}

View more solutions

Comments

[Felix Kling]

You could probably stop at Object.prototype if we really only want to get class methods. Otherwise, nice :)

[Muhammad Umer]

True, that would be useful in lots of conditions too.

[loganfsmyth]

You'll also need to .concat(Object.getOwnPropertySymbols(obj)) since getOwnPropertyNames will only return string keys. That means your example won't pick up an iterator function for example.

[loganfsmyth]

Don't forget Object.getOwnPropertySymbols too!

[antitoxic]

Nice solution. If you want to remove builtin things like __defineGetter__ you can do while ((obj = Object.getPrototypeOf(obj)) && obj != Object.prototype)

[AlexV]

Nice, but obj inside the filter is null. If it wasn't the while will never exit, right :)

[Muhammad Umer]

Yes, when obj becomes undefined while loop stops

[Wicharek]

That's a great snippet and it works. However, a small caveat: it might not work as expected if object has properties defined by Object.defineProperty or es5-style get propertyName() { }. The problem is here typeof obj[p] === 'function'. The thing is property obj[p] getter will actually get called, but with incorrect this. So if property getter uses this it will lead to unexpected results, e.g. crashes. Solution - here typeof obj[p] === 'function' instead of obj use the original one passed to this getAllMethods (store it in local variable).

[Jules]

Don't miss out the extra brackets in @antitoxic's suggestion... otherwise you start getting the prototype of true, which is apparently not null, so you go back to true again the next iteration...

[DKebler]

Ignore your linter about the assignment in while statement as it is needed

[chrismarx]

This solution returns all the internal methods that I doubt people want, how would you get just the declared methods?

[Muhammad Umer]

@Wicharek can you give an example

[Wicharek]

@MuhammadUmer here is the code I have successfully used in one of my projects

[thrice801]

These were close but still have issues with getters, namely everytime you are checking the getter you are actually calling it, which is a recipe for potential disaster, and was blowing up when I was trying to implement. Here is a version fixing that gist.github.com/jasonayre/5d9ebd64299bf69c8637a9e03e33a3fb

[Vectorjohn]

How can this work? When the loop exits, obj is falsy, so your filter will always fail because of the typeof obj[e] == 'function' reference. You need to restore the original value of obj after the loop. Actual sample output: "TypeError: Cannot read property 'defineGetter' of null"

[Muhammad Umer]

I'm not sure what I understand, if obj has prop a which is a function then obj.a will be true.

[Magico]

Nice function works great, it there the possibility to list ONLY public methods?