noRedundantUseStrict

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JavaScript (and super languages)

Summary

Rule available since: v1.0.0
Diagnostic Category: lint/suspicious/noRedundantUseStrict
This rule is recommended, which means is enabled by default.
This rule has a safe fix.
The default severity of this rule is warning.

How to configure

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{
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  "linter": {
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    "rules": {
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      "suspicious": {
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        "noRedundantUseStrict": "error"
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      }
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    }
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  }
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}

Description

Prevents from having redundant "use strict".

The directive "use strict" isn’t needed in .mjs files, or in .js files inside projects where the package.json defines library as module:

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{
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   "type": "module"
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}

Instead, .cjs files are considered “scripts” and the directive "use strict" is accepted and advised.

Examples

Invalid

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"use strict";
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function foo() {
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   "use strict";
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}

code-block.cjs:3:3 lint/suspicious/noRedundantUseStrict  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Redundant use strict directive.
  
    1 │ “use strict”;
    2 │ function foo() {
  > 3 │  	“use strict”;
      │  	^^^^^^^^^^^^^
    4 │ }
    5 │ 
  
  ℹ This outer use strict directive already enables strict mode.
  
  > 1 │ “use strict”;
      │ ^^^^^^^^^^^^^
    2 │ function foo() {
    3 │  	“use strict”;
  
  ℹ Safe fix: Remove the redundant use strict directive.
  
    3 │ ·→ “use·strict”;
      │    -------------

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"use strict";
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"use strict";
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function foo() {
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}

code-block.cjs:2:1 lint/suspicious/noRedundantUseStrict  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Redundant use strict directive.
  
    1 │ “use strict”;
  > 2 │ “use strict”;
      │ ^^^^^^^^^^^^^
    3 │ 
    4 │ function foo() {
  
  ℹ This outer use strict directive already enables strict mode.
  
  > 1 │ “use strict”;
      │ ^^^^^^^^^^^^^
    2 │ “use strict”;
    3 │ 
  
  ℹ Safe fix: Remove the redundant use strict directive.
  
    2 │ “use·strict”;
      │ -------------

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function foo() {
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"use strict";
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"use strict";
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}

code-block.cjs:3:1 lint/suspicious/noRedundantUseStrict  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Redundant use strict directive.
  
    1 │ function foo() {
    2 │ “use strict”;
  > 3 │ “use strict”;
      │ ^^^^^^^^^^^^^
    4 │ }
    5 │ 
  
  ℹ This outer use strict directive already enables strict mode.
  
    1 │ function foo() {
  > 2 │ “use strict”;
      │ ^^^^^^^^^^^^^
    3 │ “use strict”;
    4 │ }
  
  ℹ Safe fix: Remove the redundant use strict directive.
  
    3 │ “use·strict”;
      │ -------------

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class C1 {
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  test() {
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    "use strict";
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  }
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}

code-block.cjs:3:3 lint/suspicious/noRedundantUseStrict  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Redundant use strict directive.
  
    1 │ class C1 {
    2 │ 	test() {
  > 3 │ 		“use strict”;
      │ 		^^^^^^^^^^^^^
    4 │ 	}
    5 │ }
  
  ℹ All parts of a class’s body are already in strict mode.
  
  > 1 │ class C1 {
      │ ^^^^^^^^^^
  > 2 │ 	test() {
  > 3 │ 		“use strict”;
  > 4 │ 	}
  > 5 │ }
      │ ^
    6 │ 
  
  ℹ Safe fix: Remove the redundant use strict directive.
  
    3 │ → → “use·strict”;
      │     -------------

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const C2 = class {
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  test() {
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    "use strict";
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  }
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};

code-block.cjs:3:3 lint/suspicious/noRedundantUseStrict  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Redundant use strict directive.
  
    1 │ const C2 = class {
    2 │ 	test() {
  > 3 │ 		“use strict”;
      │ 		^^^^^^^^^^^^^
    4 │ 	}
    5 │ };
  
  ℹ All parts of a class’s body are already in strict mode.
  
  > 1 │ const C2 = class {
      │            ^^^^^^^
  > 2 │ 	test() {
  > 3 │ 		“use strict”;
  > 4 │ 	}
  > 5 │ };
      │ ^
    6 │ 
  
  ℹ Safe fix: Remove the redundant use strict directive.
  
    3 │ → → “use·strict”;
      │     -------------

Valid

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function foo() {
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}

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 function foo() {
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    "use strict";
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}
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function bar() {
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    "use strict";
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}