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Common Pitfalls and Mistakes

This guide covers common mistakes developers make when writing Neutron code and how to avoid them.

1. Using .length Instead of .length

❌ Wrong

var text = "Hello";
say(text.length);  // RuntimeError: Can only call functions and classes

var arr = [1, 2, 3];
say(arr.length);   // RuntimeError: Can only call functions and classes

✅ Correct

var text = "Hello";
say(text.length);    // 5

var arr = [1, 2, 3];
say(arr.length);     // 3

Why This Matters

In Neutron, .length is a property, not a method. Attempting to call it as a function () will result in a runtime error.

Key Points: - Strings have a .length property - Arrays have a .length property
- Do NOT add parentheses: use .length, not .length

String Interpolation

var items = [1, 2, 3, 4, 5];
say("Total items: ${items.length}");  // ✅ Correct

// NOT this:
say("Total items: ${items.length}");  // ❌ Error

In Loops

var data = [10, 20, 30, 40];
var i = 0;
while (i < data.length) {  // ✅ Correct
    say(data[i]);
    i = i + 1;
}

2. Forgetting to use Modules

❌ Wrong

// Trying to use sys functions without useing
var content = sys.read("file.txt");  // Error: Undefined variable 'sys'

✅ Correct

use sys;

var content = sys.read("file.txt");
say(content);

Common Module Names

  • use sys; - File I/O and system operations
  • use json; - JSON parsing and manipulation
  • use http; - HTTP client and server
  • use regex; - Regular expressions
  • use math; - Mathematical functions
  • use arrays; - Array utilities
  • use time; - Time and date functions
  • use fmt; - Type conversion and formatting

3. Incorrect String Concatenation in Loops

❌ Inefficient

var result = "";
var i = 0;
while (i < 1000) {
    result = result + "text";  // Creates many intermediate strings
    i = i + 1;
}

✅ Better (Use Array Join)

var parts = [];
var i = 0;
while (i < 1000) {
    parts.push("text");
    i = i + 1;
}
var result = parts.join("");  // More efficient

4. Modifying Arrays While Iterating

❌ Dangerous

var items = [1, 2, 3, 4, 5];
var i = 0;
while (i < items.length) {
    if (items[i] > 2) {
        items.pop();  // Modifying array during iteration
    }
    i = i + 1;
}

✅ Safer Approach

var items = [1, 2, 3, 4, 5];
var filtered = [];
var i = 0;
while (i < items.length) {
    if (items[i] <= 2) {
        filtered.push(items[i]);
    }
    i = i + 1;
}
items = filtered;

Or use the .filter() method: 

var items = [1, 2, 3, 4, 5];
items = items.filter(fun(x) { return x <= 2; });

5. Mixing Up Assignment = and Comparison ==

❌ Wrong

var x = 10;
if (x = 5) {  // This assigns 5 to x, not comparing!
    say("x is 5");
}

✅ Correct

var x = 10;
if (x == 5) {  // Comparison
    say("x is 5");
}

6. Not Checking Return Values

❌ Risky

use sys;

var content = sys.read("missing.txt");  // Returns nil if file doesn't exist
say(content.length);  // Error if content is nil

✅ Safe

use sys;

var content = sys.read("missing.txt");
if (content != nil) {
    say("File length: ${content.length}");
} else {
    say("File not found");
}

7. Incorrect JSON Access

❌ Wrong

use json;

var data = json.parse('{"name": "Alice", "age": 30}');
say(data.name);  // May not work with bracket syntax in some contexts

✅ Correct

use json;

var data = json.parse('{"name": "Alice", "age": 30}');
say(data["name"]);  // Always use bracket notation for JSON objects
say(data["age"]);

8. Forgetting return in Functions

❌ Returns nil Unexpectedly

fun add(a, b) {
    a + b;  // Result is calculated but not returned!
}

var result = add(5, 3);
say(result);  // nil

✅ Correct

fun add(a, b) {
    return a + b;
}

var result = add(5, 3);
say(result);  // 8

9. Using Wrong Comparison Operators

Equality Operators

  • == - Equals
  • != - Not equals
  • < - Less than
  • > - Greater than
  • <= - Less than or equal
  • >= - Greater than or equal

Logical Operators

  • and - Logical AND (not &&)
  • or - Logical OR (not ||)
  • not - Logical NOT (not !)

❌ Wrong

if (x > 5 && y < 10) {  // Syntax error
    say("Valid range");
}

✅ Correct

if (x > 5 and y < 10) {
    say("Valid range");
}

10. Incorrect Module Function Calls

Some functions require the module prefix, even if useed:

❌ Wrong

use arrays;

var arr = [3, 1, 2];
arr.sort();  // Error: sort is not a method

✅ Correct

use arrays;

var arr = [3, 1, 2];
arrays.sort(arr);  // Pass array as parameter
say(arr);  // [1, 2, 3]

Note: Built-in array methods like .push(), .pop(), .map(), .filter() work directly on arrays. Module functions like arrays.sort() require the module prefix.

8. HTTP Handler Must Return a Value

When using http.createServer(handler), your handler function must return a value. If it returns nil (or nothing), the server will respond with a 500 Internal Server Error.

❌ Wrong

fun handler(req) {
    say("Request received");
    // No return statement -> returns nil
}

✅ Correct

fun handler(req) {
    say("Request received");
    return "OK"; // Returns string body
}

Quick Reference Checklist

  • Use .length (property) not .length (method call)
  • use all required modules at the top of your file
  • Use == for comparison, = for assignment
  • Use and/or/not for logical operations
  • Always use return to return values from functions
  • Check for nil before accessing properties
  • Use bracket notation obj["key"] for JSON objects
  • Don't modify arrays while iterating over them
  • Remember which operations are methods vs. module functions

See Also