no-unused-expressions

A statement such as server.close; or count === 0; computes a value and does nothing with it. It's usually a mistake, like a call that's missing its parentheses, or a === where you meant =.

Calls, new, assignments, await, yield, delete, void, i++, and directives such as "use strict" do something, so they aren't reported. Neither are isReady && start(); and isReady ? start() : wait();, which are short ways to write an if.

Configure#

Off by default. To turn it on:

bunfig.toml

[lint.rules]
"no-unused-expressions" = "error" # or "warn"

Every option, set to its default:

bunfig.toml

[lint.rules]
"no-unused-expressions" = ["error", {
  allowShortCircuit = true,
  allowTernary = true,
  allowTaggedTemplates = false,
  enforceForJSX = false,
  ignoreDirectives = false,
}]

Examples#

example.js3 problems

server.close;
count === 0;
`Hello, ${name}`;

example.jsNo problems

"use strict";

server.close();
count = 0;
console.log(`Hello, ${name}`);

Options#

Option	Default	What it does
`allowShortCircuit`	`true`	Allow `a && b()`, `a \|\| b()` and `a ?? b()`, if the right side would be allowed as a statement.
`allowTernary`	`true`	Allow `a ? b() : c()`, if both branches would be allowed as statements.
`allowTaggedTemplates`	`false`	Allow a tagged template as a statement.
`enforceForJSX`	`false`	Also report a JSX element as a statement.
`ignoreDirectives`	`false`	Allow strings at the start of a file or a function when `ecmaVersion` is 3, which has no directives.

With an ESLint config file the defaults are ESLint's: every option is false.

By default, you can use && or ? : instead of an if:

example.jsNo problems

isReady && start();
isReady ? start() : wait();

The right side still has to do something:

example.js2 problems

isReady && start;
isReady ? start : wait;

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

isReady && start();

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

isReady ? start() : wait();

A tag is a function, so a tagged template can have an effect:

example.jswith { "allowTaggedTemplates": true }No problems

sql`DELETE FROM sessions`;

example.jsxwith { "enforceForJSX": true }1 problem

<Spinner />;