> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# JavaScript Hashbang Comment
A hashbang (or shebang) comment is a specialized sequence beginning with `#!` that dictates the execution environment for a script. Standardized in ECMAScript 2023 (ES14), the JavaScript engine parses this sequence as a distinct `HashbangComment` grammar element rather than a standard `SingleLineComment` (`//`). During lexical analysis, the engine strips out the hashbang and ignores its contents, provided it appears at the absolute beginning of the source text.
```javascript theme={"dark"}
#!/usr/bin/env node
const init = true;
console.log(init);
```
## Lexical Grammar and Parsing Mechanics
The JavaScript parser enforces strict rules regarding the evaluation of hashbang comments:
* **Absolute Positioning:** The `#!` sequence must occur at the exact beginning of the source text (character index 0). If it is preceded by any character, including whitespace, empty lines, or standard block/line comments, the engine will fail to parse it as a `HashbangComment` and will throw a `SyntaxError`.
* **Termination:** The comment extends from the `#!` marker until the first `LineTerminator` character (e.g., `\n`, `\r`, `\u2028`, `\u2029`). The line terminator itself is not considered part of the comment.
* **BOM Handling:** If the source file begins with a Byte Order Mark (BOM) `U+FEFF`, the ECMAScript specification dictates that the BOM is stripped prior to evaluating the first character. Therefore, a `#!` sequence immediately following a BOM remains syntactically valid.
* **Contextual Validity:** Hashbang comments are only recognized at the top level of a `Script` or `Module` goal symbol. Because `eval()` parses its input string using the `Script` goal, an optional hashbang is permitted at the beginning of the evaluated string. However, hashbangs are strictly invalid and will yield a `SyntaxError` if passed into dynamic code generation contexts that parse using the `FunctionBody` goal, such as the `Function` constructor.
```javascript theme={"dark"}
#!/usr/bin/env node
// Valid: Hashbang is at character index 0
let x = 10;
```
```javascript theme={"dark"}
#!/usr/bin/env node
// Invalid: Preceded by a space character at index 0 (Throws SyntaxError)
let y = 20;
```
```javascript theme={"dark"}
// Valid: eval() parses using the Script goal, which allows hashbangs
eval("#!/usr/bin/env node\nlet z = 30;");
// Invalid: Function constructor parses using the FunctionBody goal (Throws SyntaxError)
new Function("#!/usr/bin/env node\nreturn 40;");
```
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