noDanglingUnderscore (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v2.6.3
Diagnostic Category: lint/nursery/noDanglingUnderscore
This rule doesn’t have a fix.
The default severity of this rule is information.
Sources:
- Inspired from no-underscore-dangle

How to configure

1
{
2
  "linter": {
3
    "rules": {
4
      "nursery": {
5
        "noDanglingUnderscore": "error"
6
      }
7
    }
8
  }
9
}

Description

Disallow names that start or end with an underscore.

An underscore at the start or end of a name, as in _count or count_, is often used to say that something is private and shouldn’t be used outside of its own code. The underscore doesn’t change how the code works: a property named _id can still be read and changed from anywhere. Readers may wrongly assume the underscore has some meaning to the language.

This rule reports:

variable names, such as const _count = 0;
names of function declarations, such as function _init() {};
property names written after a dot, such as user._id or this._count.

These names are allowed:

_ on its own, which is often the name of a utility library;
__proto__ when written after a dot;
variables created by destructuring, such as const { _id } = user;
function parameters and names of function expressions;
names written where a class member or object key is declared, such as class A { _reset() {} };
private class members, whose names start with #, such as this.#_count.

Examples

Invalid

1
let _count = 0;

code-block.js:1:5 lint/nursery/noDanglingUnderscore ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ The name _count starts with an underscore.
  
  > 1 │ let _count = 0;
      │     ^^^^^^
    2 │ 
  
  ℹ An underscore at the start or end of a name is often used to mark something as private, but it doesn't change how the code works, so it can mislead readers.
  
  ℹ Remove the underscore from the name. For a class member that must stay private, start its name with # instead.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.

1
function init_() {}

code-block.js:1:10 lint/nursery/noDanglingUnderscore ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ The name init_ ends with an underscore.
  
  > 1 │ function init_() {}
      │          ^^^^^
    2 │ 
  
  ℹ An underscore at the start or end of a name is often used to mark something as private, but it doesn't change how the code works, so it can mislead readers.
  
  ℹ Remove the underscore from the name. For a class member that must stay private, start its name with # instead.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.

1
user._id;

code-block.js:1:6 lint/nursery/noDanglingUnderscore ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ The name _id starts with an underscore.
  
  > 1 │ user._id;
      │      ^^^
    2 │ 
  
  ℹ An underscore at the start or end of a name is often used to mark something as private, but it doesn't change how the code works, so it can mislead readers.
  
  ℹ Remove the underscore from the name. For a class member that must stay private, start its name with # instead.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.

1
this._count = 0;

code-block.js:1:6 lint/nursery/noDanglingUnderscore ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ The name _count starts with an underscore.
  
  > 1 │ this._count = 0;
      │      ^^^^^^
    2 │ 
  
  ℹ An underscore at the start or end of a name is often used to mark something as private, but it doesn't change how the code works, so it can mislead readers.
  
  ℹ Remove the underscore from the name. For a class member that must stay private, start its name with # instead.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.

Valid

1
let count = 0;
2
const _ = require("underscore");
3
const { _id } = user;
4
function init(_options) {}
5
const handler = function _handler() {};
6
obj.__proto__;
7

8
class Counter {
9
    #count = 0;
10
    _reset() {}
11
}