green-code-initiative · utarwyn · May 31, 2025 · May 20, 2025
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- [#400](https://github.com/green-code-initiative/creedengo-rules-specifications/pull/400) Add rule GCI535 - Prefer usage of Intl.NumberFormat
+
 ### Changed
 
 - Correction of various typos in rules documentations

diff --git a/RULES.md b/RULES.md
@@ -1,10 +1,10 @@
 # Rules
 
 - [Rules](#rules)
-  - [Rules support matrix by techno](#rules-support-matrix-by-techno)
-  - [Rules to be reworked / measured / clarified](#rules-to-be-reworked--measured--clarified)
-  - [Deprecated rules](#deprecated-rules)
-  - [Refused / Deleted rules](#refused--deleted-rules)
+    - [Rules support matrix by techno](#rules-support-matrix-by-techno)
+    - [Rules to be reworked / measured / clarified](#rules-to-be-reworked--measured--clarified)
+    - [Deprecated rules](#deprecated-rules)
+    - [Refused / Deleted rules](#refused--deleted-rules)
 
 ## Rules support matrix by techno
 
@@ -83,6 +83,7 @@ Some are applicable for different technologies.
 |          | Resize images browser-side                                        | Do not resize images using the HEIGHT and WIDTH attributes of the HTML code. This approach requires transferring these images to their original size, wasting bandwidth and CPU cycles.                                                                                                                                                                                                                                       | [cnumr best practices (3rd edition) BP_034](https://github.com/cnumr/best-practices/blob/main/chapters/BP_034_fr.md)                                                    | 🚫   | 🚫  | 🚧 | 🚫     | 🚫   | 🚫 | 🚀   |
 |          | Modify the DOM when traversing it                                 | Modifying the DOM (Document Object Model) as you traverse it can lead to situations where the loop becomes very resource-intensive, especially CPU cycles.                                                                                                                                                                                                                                                                    | [cnumr best practices (3rd edition) BP_041](https://github.com/cnumr/best-practices/blob/main/chapters/BP_041_fr.md)                                                    | 🚫   | 🚫  | 🚧 | 🚫     | 🚫   | 🚫 | 🚫   |
 |          | Edit DOM elements to make it invisible                            | When an element of the Document Object Model (DOM) needs to be modified by several properties, each change in style or content will generate a repaint or reflow.                                                                                                                                                                                                                                                             | [cnumr best practices (3rd edition) BP_042](https://github.com/cnumr/best-practices/blob/main/chapters/BP_042_fr.md)                                                    | 🚫   | 🚫  | 🚀 | 🚫     | 🚫   | 🚫 | 🚫   |
+| GCI535   | Use native Intl.NumberFormat to format numbers                    | There's no need to use a library to display formatted numbers in a recent browser. Use Intl.NumberFormat for that use case.                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                         | 🚫   | 🚫  | 🚧 | 🚫     | 🚫   | 🚫 | 🚫   |
 
 ## Rules to be reworked / measured / clarified
 
@@ -109,7 +110,6 @@ will be completely deleted in next releases and moved to bottom deleted rules ar
 |----------|------------|-----------------------------------|-----------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 | EC34     | Java / PHP | Using try...catch...finally calls | Implementation is too simple (only detection of presence of "try" statement) AND replaced by `GCI35` rule | Github discussion with measures : [general/java](https://github.com/green-code-initiative/creedengo-rules-specifications/pull/128) / [php](https://github.com/green-code-initiative/creedengo-php/pull/10) |
 
-
 ## Refused / Deleted rules
 
 This table lists rules proposed by the community but refused or/and deleted in creedengo plugins with the justification.

diff --git a/src/main/rules/GCI535/GCI535.json b/src/main/rules/GCI535/GCI535.json
@@ -0,0 +1,25 @@
+{
+  "title": "Using external libraries for number format should be avoided in favor or Intl.NumberFormat",
+  "type": "CODE_SMELL",
+  "code": {
+    "impacts": {
+      "RELIABILITY": "MEDIUM"
+    },
+    "attribute": "EFFICIENT"
+  },
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "15min"
+  },
+  "tags": [
+    "creedengo",
+    "eco-design",
+    "performance"
+  ],
+  "defaultSeverity": "Major",
+  "compatibleLanguages": [
+    "JAVASCRIPT",
+    "TYPESCRIPT"
+  ]
+}
diff --git a/src/main/rules/GCI535/javascript/GCI535.asciidoc b/src/main/rules/GCI535/javascript/GCI535.asciidoc
@@ -0,0 +1,56 @@
+:!sectids:
+
+== Why is this an issue?
+
+Importing an external library for lightweight operations increases overall size of the program.
+Using native methods instead reduces the amount of memory and storage to run and store the application.
+This is especially critical in environments with limited resources, such as on mobile devices or in web applications
+where bandwidth and download times matter.
+
+Smaller programs generally have better runtime performance.
+Reducing the number of unnecessary modules minimizes the amount of code that needs to be interpreted or compiled,
+leading to faster execution and improved overall performance.
+
+== Examples
+
+*Example with the https://numbrojs.com/[numbro] library, when you use `format` method.*
+
+[source,js]
+----
+// Example with numbro (not compliant)
+import numbro from "numbro";
+
+numbro.setLanguage('en-GB');
+var string = numbro(1000).format({
+  thousandSeparated: true,
+}); // '1,000'
+
+// Example with numerable (not compliant)
+import { format } from "numerable";
+format(1000, '0,0');
+
+// Example with Intl (compliant)
+new Intl.NumberFormat("en-GB").format(1000); // '1,000'
+----
+
+== Limitations
+
+As for now, only two libraries are handled by this rule :
+
+- https://numbrojs.com/[numbro]
+- https://numerablejs.com/lander[numerable]
+
+
+Some candidates for the future developments are :
+
+- https://github.com/Mottie/javascript-number-formatter[javascript-number-formatter]
+- https://www.npmjs.com/package/numerable[numeraljs]
+- https://formatjs.github.io/[formatjs]
+
+It’s more likely this rule won’t ever be exhaustive.
+
+== Resources
+
+=== Documentation
+
+- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/NumberFormat[Mozilla Web Technology for Developers]