JavaScript Interview Questions: Function Declaration vs Expression

Understanding the difference between function declaration and function expression is a fundamental concept in JavaScript, especially when preparing for a JavaScript interview in the context of web development. These two aspects play a significant role in how functions are written and how they behave concerning hoisting, making them essential to grasp.

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To begin, let's explore the terminology of "declaration" and "expression" in JavaScript. The confusion often arises because the term "expression" doesn't entirely capture the nature of a function. Think of an expression as merely assigning something, like assigning a function to a variable. In JavaScript, a function is a value of its own type, much like a string or number. Thus, a function expression entails assigning a function to a variable, while a declaration is explicitly defining a function using the traditional syntax.

Now, let's delve into hoisting and its interaction with these function types. When JavaScript code is executed, it undergoes two phases: a scanning phase and an execution phase. During the scanning phase, the engine identifies function declarations and stores them in memory. This allows functions to be accessible from anywhere in the code, regardless of where they were declared. On the other hand, variables declared with var are also hoisted but initialized with the value undefined. In contrast, variables declared with let and const are not hoisted at all and remain in a "dead zone" until they are assigned a value.

One essential point to remember is that when you use a function expression and assign a function to a variable, you can't use that function until the execution phase reaches the line of code with the actual assignment. This can be quite perplexing but is a crucial behavior to understand for JavaScript developers, especially during interviews.

Considering the JavaScript interview context, it is highly recommended not to use functions before their declarations in the code, as this can lead to confusion and potential errors. Being well-versed in the differences between function declaration and function expression, as well as their interactions with hoisting, will undoubtedly impress interviewers and showcase your strong understanding of JavaScript fundamentals.