In Typescript / Javascript, given the class:
class T {
method_a() {
console.log('Inside method a');
}
}
I need to access the (unbounded) method like this, but it gots undefined instead:
const method_to_be_passed = T.method_a; // undefined
In python, I would get the unbound method like this:
class P:
def method_a():
print('inside method a')
the_method = P.method_a # <function P.method_a at 0x7fee48eff310>
And then I can bound it to an object to call it. How would I do the equivalent in TS / JS?
The typescript compiler says the following error:
error TS2339: Property 'method_a' does not exist on type 'typeof T'.
In JavaScript, instance methods of a class can be accessed via the class's prototype:
class T {
method_a() {
console.log('Inside method a');
}
}
const method_to_be_passed = T.prototype.method_a;
Note that if you reference the this instance of your class inside your method, you will need to bind your method to some instance of class T like so:
const method_to_be_passed = T.prototype.method_a.bind(some_instance_of_T);
Remember that a class definition in JavaScript/TypeScript is just syntactic sugar (i.e. just shorthand) for defining a constructor-function and prototype properties:
So this
class T {
val: number = 1;
method_a() {
console.log('Inside method a');
}
}
(I added the val property as an additional example)
Is equivalent to this:
function T() {
this.val = 1;
}
T.prototype.method_a = function() {
console.log('Inside method a');
};
Therefore, to get a reference to the method_a function you need to use the prototype qualifier:
const method_to_be_passed = T.prototype.method_a;
This is different to other languages like Python, C#, C++, etc that let you reference both instance and non-instance (i.e. static) of a type without additional qualification (of course you cannot meaningfully use instance members without a this parameter, of course).
If you want method_a to not require a this parameter then you need to make it a static member:
class T {
val: number = 1;
static method_a() {
console.log('Inside method a');
}
}
which does allow you to do this:
const method_to_be_passed = T.method_a;
Playground example: