Javascript - How to use the constructor class in the promise resolve

Question

I have a class with define variable in the constructor, this class has a method that prints the value of the variable. So far so good. I call the class and the result is as expected. However, when I call the class in a promisse, the variable this does not exist. Could someone help me with this? Here is the code:

This code retur correct result

class classTest {
    constructor() {
        this._value = 'Value in constructor';
    }

    func() {
        return new Promise((resolve, reject) =>  {
            console.log(this._value);
            resolve();
        });
    }
};

let test = new classTest();
test.func();

This code retur TypeError: Cannot read property '_value' of undefined

class classTest {
    constructor() {
        this._value = 'Value in constructor';
    }

    func() {
        return new Promise((resolve, reject) =>  {
            console.log(this._value);
            resolve();
        });
    }
};

let test = new classTest();

const p = new Promise((resolve, reject) => {
    resolve()
});

p
    .then(test.func)
    .catch(console.error);