All created, without any known values, new variables should contain null.
This rule helps to easily distinguish non-existent properties and variables from cleaned or already created but not have value.

Object.prototype.hasOwnProperty() cannot be conveniently applied to large objects.

It also helps maintain consistency with the server side.
In queries "undefined" and "null" have different meanings:

• "undefined" - Value not set
• "null" - The value is set as void

MDN Web Docs - null
eslint - init-declarations

Combination of:
eslint - no-undefined
eslint - no-undef-init

let a;
const obj = {
    c: undefined,
};

console.log(a) // undefined
console.log(b) // undefined
console.log(obj.c) // undefined
console.log(obj.d) // undefined

let a = null;
const obj = {
    c: null,
};

console.log(a); // null
console.log(b); // undefined
console.log(obj.c); // null
console.log(obj.d); // undefined

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