npm install --save-dev eslint-plugin-method-names

In your .eslintrc:

{
  "plugins": [
    "method-names"
  ],
  "rules": {
    "method-names/method-names": [2, { regex: '^[a-z].*' }]
  }
}

Restrict class method names to a regex. It applies to methods and static properties defining a function.

With the configuration above, the following patterns are considered valid

class Foo {
  bar() {}
}

class Foo {
  bar = () => {}
}

class Foo {
  _bar = 40 + 2
}

whereas the following are considered errors

class Foo {
  _bar() {}
}

class Foo {
  _bar = () => {}
}

This is useful for enforcing conventions like "methods should never start with an underscore".