10 Interview Questions Every JavaScript Developer Should Know

It produces programs by composing mathematical functions and avoids shared state & mutable data.

• Good to hear: Pure functions/function purity; Avoid side-effects; Simple function composition; Examples of functional languages: Lisp, ML, Haskell, Erlang, Clojure, Elm, F Sharp, OCaml, et.; Mention of features that support FP: first-class functions, higher-order functions, functions as arguments/values.
• Red flags: No mention of pure functions / avoiding side-effects; Unable to provide examples of functional programming languages; Unable to identify the features of JavaScript that enable FP.

In JavaScript, prototypal inheritance is simpler & more flexible than class inheritance.

• Good to hear: Classes: create tight coupling or hierarchies/taxonomies; Prototypes: mentions of concatenative inheritance, prototype delegation, functional inheritance, object composition.
• Red Flags: No preference for prototypal inheritance & composition over class inheritance.

• Good to hear: Mentions of trouble with shared state, different things competing for the same resources, etc.; Awareness of FP’s capability to radically simplify many applications; Awareness of the differences in learning curves; Articulation of side-effects and how they impact program maintainability; Awareness that a highly functional codebase can have a steep learning curve; Awareness that a highly OOP codebase can be extremely resistant to change and very brittle compared to an equivalent FP codebase.
• Red flags: Unable to list disadvantages of one style or another.

The answer is never, or almost never. Certainly never more than one level.

Good to hear:

• Rarely, almost never, or never.
• A single level is sometimes OK, from a framework base-class such as React.Component.
• “Favor object composition over class inheritance.”

Good to hear:

• In situations where modules or functional programming don’t provide an obvious solution.
• When you need to compose objects from multiple sources.
• Any time you need inheritance.

Red flags:

• No knowledge of when to use prototypes.
• No awareness of mixins or `Object.assign()`.

Good to hear:  

• Avoid class hierarchies. 
• Avoid brittle base class problem. 
• Avoid tight coupling. 
• Avoid rigid taxonomy. 
• Avoid the gorilla banana problem. 
• Make code more flexible. 

Good to hear:  

• React is the new Canonical example of one-way data flow. Cycle.js is another popular implementation of uni-directional data flow.
• Angular is a popular framework which uses two-way binding. 

Good to hear:  

• Positive attitudes toward Microservices, despite higher initial cost vs Monolthic Apps. Despite higher initial cost, Microservices tend to perform and scale better.
• Structure the App so that services are independent from each other. But easy to bundle together as a monolithic App in the beginning.

Good to hear:  

• An understanding of what blocking means, and the performance implications. 
• An understanding of event handling, and why its important for UI code.