no-underscore-dangle

A leading or trailing underscore is a convention for "private", but it doesn't protect anything. Any code can still read and write user._id. If a member needs to be private, use a #private class member, which the language enforces.

By default the rule checks variable and function declarations and property access. It doesn't check parameters, destructured names, or method and class field names where they're defined. The name _ alone and obj.__proto__ are always allowed.

Configure#

Off by default. To turn it on:

bunfig.toml

[lint.rules]
"no-underscore-dangle" = "error" # or "warn"

Every option, set to its default:

bunfig.toml

[lint.rules]
"no-underscore-dangle" = ["error", {
  allow = [], # list of strings
  allowAfterThis = false,
  allowAfterSuper = false,
  allowAfterThisConstructor = false,
  allowFunctionParams = true,
  allowInArrayDestructuring = true,
  allowInObjectDestructuring = true,
  enforceInMethodNames = false,
  enforceInClassFields = false,
}]

Examples#

example.js3 problems

const _cache = new Map();

function load_(id) {
  return user._id === id;
}

example.jsNo problems

const cache = new Map();

function load(_request, id) {
  const [_first, second] = id.split("-");
  return _.isEqual(second, user.id);
}

Options#

Option	Default	What it does
`allow`	`[]`	Names that are never reported.
`allowAfterThis`	`false`	Allow `this._name`.
`allowAfterSuper`	`false`	Allow `super._name`.
`allowAfterThisConstructor`	`false`	Allow `this.constructor._name`.
`allowFunctionParams`	`true`	Allow `function f(_a) {}`.
`allowInArrayDestructuring`	`true`	Allow `const [_a] = list`.
`allowInObjectDestructuring`	`true`	Allow `const { _a } = obj`.
`enforceInMethodNames`	`false`	Also report method names, in classes and in object literals.
`enforceInClassFields`	`false`	Also report class field names.

An oxlint config has one more option. allowInUsingDeclarations (default false) allows using _lock = acquire().

bunfig.toml

[lint.rules]
no-underscore-dangle = ["error", { allow = ["_id"], allowAfterThis = true }]

example.jswith { "allow": ["_id"], "allowAfterThis": true }No problems

class Store {
  find(doc) {
    return this._items.get(doc._id);
  }
}

enforceInMethodNames and enforceInClassFields also apply to private members such as #_count:

example.jswith { "enforceInMethodNames": true, "enforceInClassFields": true }3 problems

class Store {
  _items = new Map();
  #_count = 0;

  _reset() {}
}

example.jswith { "allowFunctionParams": false }1 problem

function onClick(_event) {
  close();
}

When not to use it#

Turn it off if your project uses _name for unused variables or for members that are internal by agreement.