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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. 
#!/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.
#!/usr/bin/env node
// Valid: Hashbang is at character index 0
let x = 10;

 #!/usr/bin/env node
// Invalid: Preceded by a space character at index 0 (Throws SyntaxError)
let y = 20;

// 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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