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no-side-effects-notation-spec.md

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#__NO_SIDE_EFFECTS__ Notation Specification (Draft)

Overview

The #__NO_SIDE_EFFECTS__ notation is a special comment syntax used to indicate that a function is side-effect free, meaning it has no observable effects on the program state beyond returning a value. This notation allows developers and bundlers to optimize code by treating every call site of the annotated function as pure, enabling more aggressive dead code elimination.

Syntax

The #__NO_SIDE_EFFECTS__ notation is used as a comment in JavaScript code and should be placed before the function declaration. The notation follows the #__NO_SIDE_EFFECTS__ or @__NO_SIDE_EFFECTS__ keyword, enclosed within double underscores and double hashes.

/*#__NO_SIDE_EFFECTS__*/
function sideEffectFreeFunction() {
  // Function code here
}

Semantics

The #__NO_SIDE_EFFECTS__ notation provides a hint to developers and bundlers that the annotated function is side-effect free. It implies that calling this function will not produce any side effects beyond the returned value and can be safely removed during the optimization process when unused.

Side Effects

A function is considered to have side effects if it performs any action other than returning a value. Side effects include, but are not limited to, modifying variables or objects, reading or writing to files, making network requests, and interacting with the DOM.

By using the #__NO_SIDE_EFFECTS__ notation, developers assert that the annotated function does not have any such side effects.

Usage

The #__NO_SIDE_EFFECTS__ or @__NO_SIDE_EFFECTS__ notation can be used in the same ways as described in the previous specification:

1. Function Declaration

The #__NO_SIDE_EFFECTS__ notation should be used to mark functions that are known to be side-effect free. It can be applied to function declarations and function expressions.

/*#__NO_SIDE_EFFECTS__*/
function sideEffectFreeFunction() {
  // Function code here
}

var myVariable = /*#__NO_SIDE_EFFECTS__*/ function() {
  // Function code here
};

2. Const Variable Declaration

The #__NO_SIDE_EFFECTS__ notation can be used to mark const variables that has immediate function assignment.

/*#__NO_SIDE_EFFECTS__*/
const sideEffectFreeVariable = () => {
  // Function code here
};

3. Exported Function Declaration

The #__NO_SIDE_EFFECTS__ notation can be used to mark functions that are known to be side-effect free. It can be applied to exported function declarations and exported function expressions.

/*#__NO_SIDE_EFFECTS__*/
export function sideEffectFreeFunction() {
  // Function code here
}

 /*#__NO_SIDE_EFFECTS__*/ 
export const myVariable = function() {
  // Function code here
};

Compatibility

This notation is originally brought up by Rollup in rollup/rollup#5024.

Developers should consult the documentation of their chosen bundler or minifier to ensure compatibility and proper usage of this notation.

Reference to the Compatibility Table for a list of supported tools.