noUnusedExpressions

JavaScript (and super languages)

Summary

Diagnostic Category: lint/nursery/noUnusedExpressions
This rule doesn’t have a fix.
The default severity of this rule is information.
Sources:
- Same as no-unused-expressions

How to configure

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{
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  "linter": {
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    "rules": {
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      "nursery": {
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        "noUnusedExpressions": "error"
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      }
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    }
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  }
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}

Description

Disallow expression statements that are neither a function call nor an assignment.

When an expression is used as a statement, it should be explicitly clear what the intention behind the expression is. This is clear for function calls and assignments, because the call or the assignment itself is the primary intention behind the statement. For other expression kinds, the intention is much more ambiguous; it could be the expression contains side-effects that are not very explicit, but it could also be that it is an error where the author forgot to use the result of the expression, such as a forgotten return keyword, or it could point to a function that the author forgot to call.

Examples

Invalid

1
0

code-block.js:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ 0
      │ ^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
if(0) 0

code-block.js:1:7 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ if(0) 0
      │       ^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
{0}

code-block.js:1:2 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ {0}
      │  ^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
f(0), {}

code-block.js:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ f(0), {}
      │ ^^^^^^^^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
a && b()

code-block.js:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ a && b()
      │ ^^^^^^^^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
a, b()

code-block.js:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ a, b()
      │ ^^^^^^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
c = a, b

code-block.js:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ c = a, b
      │ ^^^^^^^^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
a() && function namedFunctionInExpressionContext () {f();}

code-block.js:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ a() && function namedFunctionInExpressionContext () {f();}
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
(function anIncompleteIIFE () {});

code-block.js:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ (function anIncompleteIIFE () {});
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
injectGlobal`body{ color: red; }`

code-block.js:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ injectGlobalbody&#123; color: red; &#125;
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
Set<number>

code-block.ts:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ Set<number>
      │ ^^^^^^^^^^^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
1 as number

code-block.ts:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ 1 as number
      │ ^^^^^^^^^^^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
window!

code-block.ts:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ window!
      │ ^^^^^^^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

JSX expressions are considered invalid when used as a statement too:

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<MyComponent />

code-block.jsx:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ <MyComponent />
      │ ^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

1
<></>

code-block.jsx:1:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
  > 1 │ <></>
      │ ^^^^^
    2 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

Valid

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{} // In this context, this is a block statement, not an object literal
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{ myLabel: foo() } // In this context, this is a block statement with a label and expression, not an object literal
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function namedFunctionDeclaration () {}
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(function aGenuineIIFE () {}());
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f()
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a = 0
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new C
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delete a.b
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void a

Handling of Directives

Any stand-alone string at the start of a script, module, or function is considered a directive and is therefore allowed.

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"use strict";
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"use asm"
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"use stricter";
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"use babel"
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"any other strings like this in the directive prologue";
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"this is still the directive prologue";
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function foo() {
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    "bar";
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}
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class Foo {
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    someMethod() {
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        "use strict";
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    }
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}

The following are not considered valid directives:

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doSomething();
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"use strict"; // this isn't in a directive prologue, because there is a non-directive statement before it

code-block.js:2:1 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
    1 │ doSomething();
  > 2 │ “use strict”; // this isn’t in a directive prologue, because there is a non-directive statement before it
      │ ^^^^^^^^^^^^^
    3 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

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function foo() {
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    "bar" + 1;
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}

code-block.js:2:5 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
    1 │ function foo() {
  > 2 │     “bar” + 1;
      │     ^^^^^^^^^
    3 │ }
    4 │ 
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.

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class Foo {
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    static {
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        "use strict"; // class static blocks do not have directive prologues
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    }
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}

code-block.js:3:9 lint/nursery/noUnusedExpressions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Expected an assignment or function call but found an expression instead.
  
    1 │ class Foo {
    2 │     static {
  > 3 │         “use strict”; // class static blocks do not have directive prologues
      │         ^^^^^^^^^^^^^
    4 │     }
    5 │ }
  
  ℹ This expression may be unintentionally unused or it might be a function that you forgot to call.