JavaScript Compatibility Issues Quiz: Polyfills and Fallbacks Quiz

Explore your understanding of JavaScript compatibility concerns, focusing on polyfills, fallbacks, and related techniques used to support older browsers and ensure robust cross-browser functionality. This quiz evaluates essential knowledge of legacy support strategies, common pitfalls, and current best practices in JavaScript development.

1. Identifying Polyfill Purpose

   If a developer wants to use the Array.prototype.includes method in browsers that do not support it, which approach is most appropriate?

   1. Using a CSS fallback
   2. Importing a build tool plugin
   3. Adding an Array.prototype.includes polyfill
   4. Declaring a variable with var

   Explanation: A polyfill is a script that replicates modern JavaScript functionality, such as Array.prototype.includes, for older environments where this method is missing. A CSS fallback would not add JavaScript features. Importing a build tool plugin might aid development but would not by itself provide the missing method at runtime for all browsers. Declaring a variable with var is unrelated to adding absent methods to arrays.

2. Polyfill Detection Technique

   What is a recommended way to check if a method like Promise is natively available before applying a polyfill?

   1. typeof polyfill === 'function'
   2. if (Promise != applyPolyfill)
   3. Promise.exist()
   4. if (typeof Promise === 'undefined') { /* apply polyfill */ }

   Explanation: Checking if Promise is undefined via typeof allows the script to detect absence of native support and conditionally load a polyfill. The second option is nonsensical and would not work. Promise.exist() is not a real method. The last option checks for a variable that is not standard in this context.

3. Fallbacks in Feature Detection

   Which code snippet best demonstrates providing a fallback value if navigator.geolocation is not supported?

   1. navigator.geolocation = false;
   2. if (typeof setTimeout === 'undefined') { useDefaultLocation(); }
   3. navigator.geolocation.orDefault(useDefaultLocation());
   4. if (!navigator.geolocation) { useDefaultLocation(); }

   Explanation: Checking if navigator.geolocation exists and then providing a fallback, as shown, is a correct way to handle missing features. Setting navigator.geolocation to false risks breaking feature detection. The third option uses a non-existent method and would cause errors. The last option checks for setTimeout instead of geolocation, which isn't relevant to the feature in question.

4. Polyfill and Prototype Modification

   When implementing a polyfill for Array.prototype.map, what is a crucial step to avoid unintended side effects?

   1. Change the name of Array.prototype.map to Array.prototype.maps
   2. Set map to null after adding the polyfill
   3. Replace all instances of Array with a new object
   4. Check if Array.prototype.map already exists before defining it

   Explanation: Polyfills should only define missing functionality, so checking for Array.prototype.map helps avoid overwriting native implementations. Replacing all instances of Array or setting map to null could break code. Changing the method name would not help and would reduce compatibility with code expecting the standard map method.

5. Best Practices in Polyfill Loading

   What is a common advantage of loading polyfills conditionally instead of including them for all users?

   1. It reduces unnecessary JavaScript execution for modern browsers
   2. It prevents JavaScript errors in unsupported browsers
   3. It allows HTML fallback elements to work automatically
   4. It speeds up polyfill loading in unsupported browsers

   Explanation: By conditionally loading polyfills, modern browsers that already support a feature avoid redundant downloads and execution, leading to better performance. Conditional loading does not inherently make loading faster in old browsers. It also does not directly prevent errors (though it avoids redefining supported features). HTML fallback elements are unrelated to the mechanism of conditional polyfill loading.