JavaScript Variable Names: Valid Characters & Library Conflicts

JavaScript variable names can include letters (a-z, A-Z), numbers, underscores (_), dollar signs ($), and certain Unicode characters. For single-character variables, only letters, underscore, and dollar sign are valid, but you’ll need to avoid $ due to jQuery conflicts and _ which is often used for private variables. Careful consideration of library conventions is essential when creating extension libraries to prevent namespace conflicts.

Contents

• JavaScript Variable Naming Rules: What Characters Are Valid
• Single-Character Variable Options in JavaScript
• Common JavaScript Libraries and Their Single-Character Conventions
• Identifying and Avoiding Library Conflicts
• Best Practices for Choosing Variable Names in Extension Libraries
• Advanced Techniques: Using Symbols and Unicode in Variable Names

JavaScript Variable Naming Rules: What Characters Are Valid

JavaScript variable names, technically called identifiers, must follow specific rules defined by the ECMAScript specification. The language allows several types of characters in JavaScript variable names, but with important restrictions. According to Mozilla Developer Network, valid JavaScript variable names must start with a letter (a-z, A-Z), underscore (_), or dollar sign ($). Subsequent characters can include letters, digits (0-9), underscores, and dollar signs.

Interestingly, modern JavaScript also supports certain Unicode characters in variable names, including non-ASCII letters and digits from other languages. This means you can use characters like α, β, γ, or é in your variable names, as documented by Mathias Bynens. The specification allows for any character with the Unicode property “ID_Start” as the first character, and any character with the Unicode property “ID_Continue” in subsequent positions.

However, there are limitations to be aware of. Variable names cannot be reserved keywords like function, var, let, const, class, etc. They also cannot contain spaces or special characters like @, #, %, etc. Numbers are allowed but cannot be the first character of a variable name.

// Valid variable names
let myVariable = 42;
let _private = "hidden";
let $dollar = 100;
let αβγ = "Greek letters";
let camelCase = "style";
let snake_case = "style";

// Invalid variable names
// let 123number = "starts with number"; // Error
// let my-variable = "contains hyphen"; // Error
// let function = "reserved keyword"; // Error
// let my variable = "contains space"; // Error

When creating extension libraries, understanding these basic rules is crucial, especially when you’re considering single-character variable names that might conflict with existing JavaScript libraries.

Single-Character Variable Options in JavaScript

When focusing specifically on single-character JavaScript variable names, the options become significantly limited. As mentioned earlier, only four types of single-character variables are syntactically valid: letters (a-z, A-Z), underscore (_), and dollar sign ($). This limitation creates challenges for extension library developers who want to use concise, memorable variable names without conflicts.

The letters a through z (both lowercase and uppercase) represent 52 possible single-character variable names. The underscore and dollar sign add two more options, bringing the total to 54 possible single-character variable names. While this might seem like plenty of options, several of these are commonly used or reserved for specific purposes.

For example, the dollar sign ($) is universally associated with jQuery and many other JavaScript libraries. The underscore (_) is frequently used to denote private variables or special properties in various frameworks. Single letters like a, b, c are often used as loop counters or temporary variables in local scopes.

// All valid single-character variable names
let a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z;
let A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z;
let _;
let $;

According to SQLpey JavaScript Guide, the restriction to these specific single-character symbols is due to JavaScript’s syntactic parsing requirements. The language needs to distinguish between variable names and other tokens, which is why characters like @, #, %, etc., are not allowed in variable names at all.

This limitation means that extension library developers must either use multi-character variable names or carefully select from the limited pool of single-character options that don’t conflict with existing libraries or common coding conventions.

Common JavaScript Libraries and Their Single-Character Conventions

Understanding the single-character conventions used by popular JavaScript libraries is essential for avoiding conflicts when creating extension libraries. Two of the most influential libraries that established these conventions are jQuery and Prototype.js.

jQuery Conventions

jQuery famously uses the dollar sign ($) as its primary namespace. This convention dates back to jQuery’s early days and has become so widespread that many developers automatically associate $ with jQuery. When jQuery is loaded on a page, it creates a global variable $ that references the jQuery function.

// jQuery usage
$("div").hide(); // Selects and hides all div elements
$.ajax("/api/data"); // Makes an AJAX request

jQuery also provides a method called noConflict() that allows developers to relinquish control of the $ variable, which is particularly useful when other libraries like Prototype.js are also using $.

Prototype.js Conventions

Prototype.js, an older JavaScript library that predates jQuery, established a more extensive set of single-character conventions. In addition to using $ for its namespace, Prototype.js introduced several other single-character variables:

• $: The main namespace for Prototype functions

• $$: CSS selector function (returns multiple elements)$$

• $F: Gets the value of a form field
• $A: Converts an array-like object to a true array
• $H: Converts an object to a hash (associative array)
• $R: Creates a range object
• $w: Splits a string into an array of words

As noted in Prototype.js documentation, these conventions were designed to provide concise, powerful APIs that would feel natural to Ruby developers who were familiar with these patterns.

// Prototype.js usage
$("myElement").hide(); // Hides an element with ID 'myElement'
$$("div.highlight"); // Returns all div elements with class 'highlight'
$F("username"); // Gets the value of the form field with ID 'username'

According to discussions in Prototype.js Google Groups, these conventions were intentionally chosen to be concise and powerful, but they also created significant namespace pollution that made it difficult to use Prototype alongside other libraries.

Other Library Conventions

Other popular JavaScript libraries have adopted different single-character conventions:

• Underscore.js and Lodash use _ as their primary namespace
• Moment.js uses moment (not single-character, but worth noting)
• D3.js uses d3 as its namespace

When creating extension libraries, understanding these established conventions is crucial. The single-character space is effectively crowded, with $ being claimed by jQuery, _ being used by many libraries, and various letters being used in specific contexts by different frameworks.

Identifying and Avoiding Library Conflicts

When developing extension libraries, avoiding conflicts with existing JavaScript libraries is paramount to ensuring your library works reliably across different environments. Several strategies can help you identify potential conflicts and resolve them effectively.

Detecting Existing Library Usage

Before loading your extension library, you should check which libraries are already present and which single-character variables they’re using. This detection allows you to make informed decisions about your own variable naming conventions.

// Check for common library conflicts
const libraries = {
 jquery: typeof $ !== 'undefined' && $.fn && $.fn.jquery,
 prototype: typeof $ !== 'undefined' && $$ !== 'undefined' && $A !== 'undefined',
 underscore: typeof _ !== 'undefined' && _.isObject && _.isObject(_)
};

console.log('Detected libraries:', libraries);

Using jQuery’s noConflict() Method

jQuery provides a built-in mechanism to resolve conflicts with other libraries. The noConflict() method allows you to control the $ variable and assign jQuery to a different variable name:

// Standard jQuery loading
jQuery(document).ready(function($) {
 // Code using $ for jQuery
});

// jQuery noConflict pattern
jQuery(document).ready(function(jQ) {
 // Use jQ instead of $ for jQuery
 jQ("div").hide();
});

According to jQuery’s official documentation, this pattern is recommended when jQuery is used alongside other libraries that also use $ as their namespace.

IIFE Wrapper Pattern

A more comprehensive approach is to wrap your entire extension library in an Immediately Invoked Function Expression (IIFE). This pattern creates a private scope that doesn’t pollute the global namespace:

// IIFE wrapper for extension library
(function(window, document, undefined) {
 'use strict';
 
 // Library code here
 // You can use $ safely if jQuery isn't needed
 // Or use a different convention like lib$
 
 // Expose library to global namespace
 window.MyExtension = {
 // Public API
 };
})(window, document);

This pattern prevents conflicts by keeping all internal variables within the private scope of the IIFE. Only the parts you explicitly expose to the global namespace (like window.MyExtension in the example) will be available to other scripts.

Custom Namespace Strategy

Instead of using single-character variables, you can create a custom namespace for your extension library. This approach provides better organization and reduces the chance of conflicts:

// Create a unique namespace
const MyExtension = {
 // Use descriptive multi-character names
 init: function() {
 // Library initialization
 },
 
 config: {
 // Configuration options
 },
 
 utils: {
 // Utility functions
 }
};

// Public API
window.MyExtension = MyExtension;

As discussed in Sreyas’s blog on JavaScript library conflicts, this namespace approach is more scalable and maintainable than using single-character variables, especially for larger libraries.

Conflict Resolution in Practice

When working with existing pages that might have conflicting libraries, you can use conditional loading techniques:

// Conditional loading with conflict resolution
(function() {
 // Check for conflicts
 const conflicts = [];
 
 if (typeof $ !== 'undefined' && $ !== jQuery) {
 conflicts.push('$ is already in use');
 }
 
 if (typeof _ !== 'undefined' && !_.isObject) {
 conflicts.push('_ is already in use');
 }
 
 if (conflicts.length > 0) {
 console.warn('Potential library conflicts detected:', conflicts);
 // Optionally load a noConflict version of your library
 } else {
 // Load standard version
 }
})();

By implementing these strategies, you can effectively identify and resolve conflicts between your extension library and existing JavaScript libraries, ensuring a more robust and compatible library.

Best Practices for Choosing Variable Names in Extension Libraries

Creating a successful extension library involves more than just functionality—it requires careful consideration of variable naming conventions to ensure compatibility and maintainability. Following best practices for variable naming can help you create libraries that are both powerful and respectful of other code on the page.

Avoid Single-Character Variables When Possible

While single-character variables might seem convenient, they create significant namespace pollution and conflict risks. For extension libraries, it’s generally better to use descriptive multi-character variable names that clearly indicate their purpose.

// Instead of:
function $(selector) { /* ... */ }
function _(obj) { /* ... */ }

// Use:
function querySelector(selector) { /* ... */ }
function extendObject(obj) { /* ... */ }

According to Dev Namespace Pollution Guide, descriptive naming reduces cognitive load for developers using your library and makes the code more maintainable over time.

Use Unique Prefixes or Namespaces

If you need to create multiple global variables or functions, consider using a unique prefix or namespace pattern. This approach groups related functionality while minimizing conflicts.

// Instead of multiple global variables
let utils = { /* ... */ };
let widgets = { /* ... */ };
let events = { /* ... */ };

// Use a single namespace
const MyLib = {
 utils: { /* ... */ },
 widgets: { /* ... */ },
 events: { /* ... */ }
};

The prefix should be unique enough to avoid conflicts—consider using your company name, project name, or a distinctive identifier.

Check for Reserved Words and Common Conventions

Before finalizing variable names, check against lists of JavaScript reserved words and commonly used variable names in your domain. This proactive approach can prevent subtle bugs and conflicts.

// Check against reserved words
const reservedWords = [
 'abstract', 'arguments', 'await', 'boolean', 'break', 'byte',
 'case', 'catch', 'class', 'const', 'continue', 'debugger',
 'default', 'delete', 'do', 'double', 'else', 'enum',
 'eval', 'export', 'extends', 'false', 'final', 'finally',
 'float', 'for', 'function', 'goto', 'if', 'implements',
 'import', 'in', 'instanceof', 'int', 'interface', 'let',
 'long', 'native', 'new', 'null', 'package', 'private',
 'protected', 'public', 'return', 'short', 'static', 'super',
 'switch', 'synchronized', 'this', 'throw', 'throws',
 'transient', 'true', 'try', 'typeof', 'var', 'void',
 'volatile', 'while', 'with', 'yield'
];

// Avoid common conventions
const commonConventions = ['$', '_', 'jQuery', 'React', 'angular', 'vue'];

function isSafeVariableName(name) {
 return !reservedWords.includes(name) && 
 !commonConventions.includes(name) &&
 /^[a-zA-Z_$][0-9a-zA-Z_$]*$/.test(name);
}

Use CamelCase for Public APIs

For public-facing APIs, camelCase is the most widely accepted convention in JavaScript. This style is familiar to most JavaScript developers and integrates well with existing codebases.

// Good public API naming
const MyExtension = {
 initLibrary: function() { /* ... */ },
 configureOptions: function(options) { /* ... */ },
 renderWidget: function(container) { /* ... */ },
 calculateMetrics: function(data) { /* ... */ }
};

Consider Private Variables and Closures

For internal implementation details, consider using private variables through closures or the Module pattern. This approach keeps your public API clean while allowing for complex internal implementations.

// Module pattern with private variables
const MyExtension = (function() {
 // Private variables
 let privateConfig = {};
 let internalCache = {};
 
 // Public API
 return {
 init: function() {
 // Public initialization
 },
 
 getConfig: function() {
 // Returns a copy of config
 return {...privateConfig};
 }
 };
})();

Document Your Naming Conventions

Whatever conventions you choose, document them clearly in your library’s README and API documentation. This documentation helps users understand your design decisions and makes it easier for them to contribute to your project.

# Naming Conventions

- Public APIs use camelCase
- Internal variables start with underscore (_)
- Constants are in ALL_CAPS
- Configuration objects use descriptive keys
- Event names use kebab-case

By following these best practices, you can create extension libraries that are both powerful and considerate of other code on the page, reducing the likelihood of conflicts and making your library more appealing to developers.

Advanced Techniques: Using Symbols and Unicode in Variable Names

For extension library developers who need to maximize namespace availability while maintaining clean code, advanced features like JavaScript Symbols and Unicode characters offer innovative solutions. These techniques provide ways to create unique identifiers that are less likely to conflict with existing libraries.

JavaScript Symbols: Unique Property Identifiers

ES6 introduced Symbols, a new primitive type that creates unique, immutable values. Symbols are particularly useful for creating property keys that won’t conflict with other code, even if they use the same string identifier.

// Creating unique symbols
const mySymbol = Symbol('my-extension');
const anotherSymbol = Symbol('my-extension'); // Different value

console.log(mySymbol === anotherSymbol); // false

// Using symbols as property keys
const myObject = {
 [mySymbol]: 'private value',
 publicMethod: function() {
 return this[mySymbol];
 }
};

console.log(myObject[mySymbol]); // 'private value'

According to JavaScript.info, Symbols are ideal for creating unique identifiers that won’t collide with other property names, making them perfect for extension libraries that need to store private data on objects without risking conflicts.

One advanced technique is using Symbols to extend native prototypes without polluting the global namespace:

// Safely extending Array prototype with Symbols
const arrayExtension = Symbol('array-extension');

Array.prototype[arrayExtension] = function() {
 return this.map(item => item * 2);
};

// Use the extension
const numbers = [1, 2, 3];
console.log(numbers[arrayExtension]()); // [2, 4, 6]

// No conflict with other methods
console.log(Object.getOwnPropertyNames(Array.prototype)); // Doesn't include our symbol

As noted in Dev.to article on Symbols, this approach provides encapsulation while allowing for powerful extensions to built-in types.

Unicode Characters in Variable Names

JavaScript allows Unicode characters in variable names, opening up a wide range of possibilities for unique identifiers. This feature can be particularly useful for creating distinctive variable names that are unlikely to conflict with existing code.

// Valid Unicode variable names
const π = 3.14159;
const α = 1;
const β = 2;
const λ = function(x) { return x * 2; };
const ∑ = function(arr) { return arr.reduce((a, b) => a + b, 0); };

console.log(∑([1, 2, 3, 4])); // 10

According to Mathias Bynens’ guide on JavaScript identifiers, JavaScript supports any character with the Unicode property “ID_Start” as the first character, and any character with the Unicode property “ID_Continue” in subsequent positions. This includes letters from various scripts, mathematical symbols, and other Unicode characters.

However, there are important considerations when using Unicode in variable names:

1. Readability: While Greek letters like α, β, γ might be familiar to some developers, more obscure Unicode characters can make code harder to read and maintain.

2. Input Method: Developers might have difficulty typing these characters, especially on different keyboard layouts or operating systems.

3. Searchability: Searching for variables with Unicode characters can be challenging, especially across different editors and environments.

4. Consistency: Mixing Unicode with ASCII characters can lead to inconsistencies in code style.

Combining Techniques for Maximum Safety

For maximum namespace safety, consider combining Symbols, Unicode, and conventional naming techniques:

// Extension library using multiple techniques
const MyExtension = (() => {
 // Private symbol for internal state
 const _state = Symbol('my-extension-state');
 
 // Unicode constants for common operations
 const ∂ = 'partial'; // Partial derivatives
 const ∇ = 'gradient'; // Gradient
 
 // Public API with conventional naming
 return {
 init: function(config) {
 this[_state] = { config, cache: {} };
 },
 
 // Use Unicode for distinctive method names
 [∂]: function(obj, prop) {
 // Implementation of partial derivative logic
 },
 
 [∇]: function(data) {
 // Implementation of gradient calculation
 },
 
 // Conventional methods for common operations
 calculate: function(input) {
 return this[∇](input);
 }
 };
})();

Browser Compatibility Considerations

While advanced techniques like Symbols and Unicode are powerful, it’s important to consider browser compatibility:

• Symbols are supported in all modern browsers (ES6+), but not in older browsers like IE11.
• Unicode variable names have excellent support across modern browsers but may cause issues in older environments or certain build tools.
• Always check compatibility requirements for your target audience.

Practical Recommendations

1. Use Symbols for private properties when you need to store data on objects without risking conflicts.

2. Reserve Unicode for distinctive constants rather than common variables, balancing uniqueness with readability.

3. Document your unconventional naming so other developers understand your design decisions.

4. Provide fallbacks for older browsers when using advanced features.

5. Consider transpilation if you need to support older browsers while using modern syntax.

By leveraging these advanced techniques, you can create extension libraries that minimize namespace conflicts while maintaining clean, expressive code. However, always balance innovation with practicality and consider the maintainability of your code for future developers.

Sources

1. Mozilla Developer Network — Official JavaScript documentation on variable naming rules: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Grammar_and_types
2. Mathias Bynens — Comprehensive guide on JavaScript identifiers and Unicode support: https://mathiasbynens.be/notes/javascript-identifiers
3. SQLpey JavaScript Guide — Detailed explanation of JavaScript variable naming rules: https://sqlpey.com/javascript/javascript-variable-naming-rules/
4. Tutorialspoint — Examples and explanations of valid JavaScript variable names: https://www.tutorialspoint.com/What-characters-are-valid-for-JavaScript-variable-names
5. Patel Hemil Medium — Clear rules for JavaScript variable naming with practical examples: https://patelhemil.medium.com/javascript-rules-for-valid-variable-names-3ba80ae09250
6. GeeksforGeeks — Basic rules for JavaScript variable names with examples: https://www.geeksforgeeks.org/javascript/what-characters-are-valid-for-javascript-variable-names/
7. jQuery Documentation — Official guidance on avoiding conflicts with other libraries: https://learn.jquery.com/using-jquery-core/avoid-conflicts-other-libraries/
8. Sreyas Blog — General strategies for handling JavaScript library conflicts: https://sreyas.com/blog/handling-javascript-library-conflicts/
9. Stack Overflow jQuery Prototype Conflict — Community insights on specific jQuery and Prototype conflict resolution: https://stackoverflow.com/questions/10034991/jquery-and-prototype-js-conflict-how-to-keep-jquery-as
10. Prototype.js Google Groups — Discussion about Prototype.js single-character conventions: https://groups.google.com/g/prototype-scriptaculous/c/7J26UXM_91Y
11. Makitweb — Strategies for avoiding jQuery conflicts with other libraries: https://makitweb.com/how-to-avoid-jquery-conflict-with-other-js-libraries/
12. Stack Overflow jQuery Prototype Conflict — Specific issues with Prototype conflicts: https://stackoverflow.com/questions/2189929/how-can-we-resolve-conflict-between-prototype-and-jquery
13. Stack Overflow jQuery Prototype Conflict — Detailed explanation of jQuery and Prototype conflict resolution: https://stackoverflow.com/questions/2473979/jquery-prototype-conflict
14. Wikipedia Prototype.js — Overview of Prototype.js and its conventions: https://en.wikipedia.org/wiki/Prototype_JavaScript_Framework
15. Dev.to Slikts — Using symbols for extending native prototypes safely: https://dev.to/slikts/extending-native-js-prototypes-is-not-such-a-crazy-idea-with-symbols-2h8l
16. Prototype.js Documentation — Official Prototype.js string methods and conventions: http://prototypejs.org/doc/1.6.0/string.html
17. Dev.to Micmath — Working with JavaScript symbols in practice: https://dev.to/micmath/working-with-javascript-symbols-10k8
18. Dev.to EFPage — Guide to avoiding namespace pollution in JavaScript: https://dev.to/efpage/how-to-avoid-namespace-pollution-in-javascript-3h3
19. Medium Intrinsic Blog — Benefits of using JavaScript symbols for avoiding conflicts: https://medium.com/intrinsic-blog/javascript-symbols-but-why-6b02768f4a5c

Conclusion

JavaScript variable names are governed by specific rules that allow letters, numbers, underscores, dollar signs, and certain Unicode characters. When creating extension libraries, single-character variable options are limited to letters, underscore, and dollar sign, but these are often already claimed by popular libraries. jQuery uses $ as its primary namespace, while Prototype.js employs multiple single-character variables like $, $$, $F, $A, $H, $R, and $w. To avoid conflicts, extension library developers should consider using descriptive multi-character names, implementing IIFE wrappers, creating unique namespaces, and leveraging advanced techniques like JavaScript Symbols for private properties. By following best practices and understanding established conventions, you can create extension libraries that are both powerful and respectful of other code on the page.