Get functions (methods) of a class

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.

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;