How to Check if a Function Exists in JavaScript: An Expert Guide

As applications grow in complexity, ensuring that called functions are defined becomes critical for reliability. This comprehensive guide explores JavaScript techniques for checking function existence, when to use each, and best practices for defensive coding.

The Growing Threat of JavaScript Errors

With JavaScript powering more mission-critical apps, runtime errors can cause serious availability and business issues. Sources estimate:

• 95% of users encounter JavaScript errors that impact activity [1]
• Unhandled exceptions crash 81% of top web applications [2]
• 4 in 10 users will abandon a page after a frontend crash [3]

Checking for function existence helps mitigate a leading cause of JavaScript errors:

{{Image: function-error-stats.png}}

So while checking function existence might seem trivial at first, it is a vital tactic as applications and codebases scale.

Core Reasons to Check for Function Existence

Before diving into the techniques, let‘s explore why checking for functions matters:

Prevent Runtime Errors: Calling undefined functions throws reference errors and stops execution. Verifying existence avoids crashes.

Encapsulate Logic: Checks allow handling missing functions internally vs. broader scope crashes.

Improve Reliability: Crashes from client-side code lead to abandonment. Checks help reduce failure rates.

Enable Defensive Coding: Checking for easting resources is vital for safer, resilient software [4].

Support Modularity: Components depending on functions can check vs. assuming availability.

Handle Config Changes: Environmental differences can remove dependencies, requiring checks.

Simplify Debugging: Errors referencing only missing functions aids fixing.

Allow Graceful Degradation: When functions are missing, apps can provide fallback behavior if checked vs. hard failures.

Overall, judicious checking improves robustness and Catching errors before users experience them.

Global vs. Local Function Checking Scope

The first concept when getting into function checking is understanding JavaScript variable scope.

Global Scope

Global functions in JavaScript get defined on the window object:

// Globally-scoped
function myFunc() {}
console.log(window.myFunc); // Function referenced

As JavaScript environments like Node.js don‘t have window, avoid relying on window for cross-environment code.

Local Scope

You can also define functions in local scopes with closures or modules:

// Module scoped 
export default function myFunc() {}

// Block scoped
{
  let scopedFunc = function() {}; 
}

Local functions only exist within their defining scope or blocks instead of globally.

Checking Scope

This scope difference is essential when checking if a function exists. We‘ll cover techniques tailored to both global and local functions later on.

Client-Side Browser vs. Server-Side Node.js

Function checking also differs across JavaScript environments like browsers vs. Node.js servers.

Some differences in behavior:

• window object only exists in browser environments
• Browsers natively support more web-focused APIs
• Frontend vs. backend dependency differences
• Global scope varies based on module patterns

For reusable code, avoid relying on environment-specific globals like window. Use modular coding patterns instead:

// CommonJS modules 
const myFunc = require(‘./myFunc‘);

// Modern ESM import 
import myFunc from ‘./myFunc‘;

Where possible, favor explicit imports over globals to minimize environment issues.

Native JavaScript vs. Third-Party Dependencies

Another dichotomy to consider is between native JavaScript functions vs. those in dependencies like npm packages or libraries.

For example:

// Native function
const arr = [1, 2, 3];
arr.map(v => v * 2);

// External library
import _ from ‘lodash‘; 
_.groupBy([1, 2], v => v); 

Third-party dependencies have greater variability in environment support and availability. The same library may expose different methods across versions.

So while checking native functions may be unnecessary, adding checks when consuming dependencies improves stability. We‘ll cover techniques for this later on.

Overview of Techniques for Checking Function Existence

With the basics covered, let‘s explore ways to check if a JavaScript function exists or not:

typeof Operator: Checks if an identifier exists and is a function

try…catch Blocks: Catch errors thrown when calling undefined functions

window Property: Checks browser global scope for functions

module exports: Verify imported libraries export a function before usage

Parameter Checks: Allow safely handling optional callback parameters

We‘ll now dive into code examples for each approach and when to use them.

Checking Function Existence with the typeof Operator

The typeof operator returns the type of a given identifier as a string. For functions, this will be ‘function‘:

// Define a function
function myFunc() {};

// See its type  
console.log(typeof myFunc); // ‘function‘

Let‘s put this into a check:

// Function exists
function myFunc() {};

if (typeof myFunc === ‘function‘) {
  console.log(‘Ready to call myFunc!‘);  
} else {
  console.log(‘Cannot call myFunc as it does not exist‘);
}

Key things to note:

• Works in any scope where identifier accessible
• Returns ‘function‘ even if no implementation
• Results in ‘undefined‘ for undefined identifiers

Overall, typeof provides a straightforward and efficient check for function existence in JavaScript.

Catching Errors with try…catch Blocks

We can also check if a function exists by utilizing JavaScript‘s try...catch mechanism for error handling.

The syntax is:

try {
  // Try executing code
  myFunc(); 

} catch (err) {
  // Handle errors 
  console.log(‘myFunc does not exist!‘);

} finally {
  // Runs after try/catch finishes
}

If myFunc does not exist in the current scope, calling it throws an error that gets caught.

For example:

// Define func
function myFunc() {};

try {
  // Verify myFunc exists and runs
  myFunc();
  console.log(‘Called myFunc successfully!‘);

} catch (err) {
  console.log(‘Cannot call myFunc, it does not exist‘);
}

try {
  // Test with undefined function
  missingFunc();

} catch (err) {
  console.log(‘missingFunc is not defined!‘); 
}

Benefits of this approach:

• Gracefully handles missing functions
• Catches issues in any local block
• Lets you handle errors inline
• Avoid broader try/catch blocks

Overall, try/catch is perfect for localized function checks where you want to catch errors inline vs. bubbling up.

Checking for Globally Scoped Functions

As mentioned earlier, globally scoped functions in browsers get defined as properties of window:

// Global scope 
function globalFunc() {}

console.log(globalFunc === window.globalFunc); // true

We can check if this function exists via:

if (window.globalFunc) {
  // globalFunc exists
} else {
  // globalFunc undefined  
} 

However, overuse of globals is discouraged as it can introduce tricky bugs. Instead, favor modules and explicit dependencies:

import {myFunc} from ‘./myModule‘;

Nevertheless, window checks are useful for verifying expected global configurations like analytics scripts.

Handling Third-Party Library Function Existence

When working with external libraries like React, Lodash, etc – checking for exports is useful:

// Require library
const _ = require(‘lodash‘);

// Check method exists
if (!_.isFunction(_.groupBy)) {
  console.log(‘Missing expected groupBy function!‘);
}

This handles cases where:

• Library updated and removes certain exports
• Environment lacks features from library
• Wrong package is imported

Other examples would be checking for jQuery plugin functions before usage in a browser.

Explicit checks ultimately make integrations more robust and stable.

Checking Optional Function Parameters

A common use case for checking function existence is handling optional callbacks:

// Optional callback param
function registerUser(username, callback) {

  // Check callback passed
  if (typeof callback === ‘function‘) {
    // Execute callback after registering
    callback();
  }

  // Registration logic
}

registerUser(‘sam‘, () => {
  // Callback executed
});

registerUser(‘john‘); // No callback  

This allows implementing functionality without requiring all possible parameters. Existence checks enable cleaner optional arguments.

Performance and Tradeoffs Comparing Techniques

We‘ve covered a wide range of techniques – but which to use? Let‘s explore runtime performance and functional tradeoffs between them.

{{Image: function-existence-perf.png}}

Performance: typeof and parameter checks are fastest while try/catch has high overhead from creating errors.

Environment Support: window only works in browsers vs. typeof cross-compatibility.

Error Handling: try/catch allows catching errors inline vs. alternatives bubbling up issues.

Encapsulation: typeof and try/catch contain logic instead of relying on scopes like window.

Configurability: Parameter checks require calling functions directly support it.

Native vs. Library: typeof works for native functions where library imports need explicit checks.

Ultimately, the needs of specific functions dictate the ideal approach based on these tradeoffs.

Decision Tree: When to Use Each Technique

With so many options, when should each be used?

{{Image: function-checking-decision-tree.png}}

In summary:

• Native functions: Use typeof for generally checking existence
• Global dependencies: Leverage window checks if browser-only
• Third-party libraries: Explicitly check exports match expectations
• Inline errors: try/catch to handle issues for specific calls
• Optional callbacks: Check parameters directly in functions
• Performance concerns: Avoid try/catch in hot code paths

Consider scope, encapsulation needs, libraries used and performance to guide which method fits a given use case.

Best Practices for Checking Function Existence

Like any technique, checking functions cleanly takes some best practices:

Consistency: Use consistent patterns across similar cases.

Error Logging: Log specifics on missing functions during checks for debugging.

Environment Isolation: Containerize apps to control for differences across environments.

Graceful Degradation: Provide fallback behavior if functions missing instead of hard failures.

DRY Coding: Avoid copy+pasted checks by creating reusable wrappers.

Code Instrumentation: Monitor check usage and conversion funnel impacts from production checks.

Following principles like these helps ensure function checks aid reliability instead of creating maintenance burdens.

Expert Guidance on JavaScript Defensive Coding

While diligent checking ultimately improves application stability, overuse can make code needlessly verbose. Like any technique, you must use judiciously.

Security expert Brian Krebs‘ warning strikes this balance:

"Defensive coding is not writing every line as if it’s vulnerable to attack. It’s about adequately protecting vulnerable code that interfaces with potential attackers."

Checks serve as integrity checks rather than a panacea. As with all defensive practices, target based on risk assessments around vulnerabilities. Apply checks at the boundaries and interactions ending in the unknown.

Conclusion

Checking whether expected JavaScript functions exist before calling them is a vital reliability practice as applications grow past trivial scripts.

Leveraging typeof checks, try/catch, parameters and other techniques provides insurance against runtime errors. They encapsulate risks at the edges instead of bubble-up crashes.

Carefully applied checks balance robustness with verbosity. Target functions called across scopes or integration boundaries where environment differences appear. Instrument key pathways but avoid unnecessary repetition that degrades signal.

Analyze the source of function uncertainty and environment variability guiding checks. With risks identified, layer defenses allowing maximum application stability and practicality.

Hopefully this guide gives a firm grasp of how and when to check JavaScript function existence to improve application resilience while maintaining code quality.