First-Class Functions Interview Questions in JavaScript

First-Class Functions Interview Questions in JavaScript

First-class functions are a core interview topic. Here are the most asked questions with answers.

Q1: What does it mean that functions are first-class citizens in JavaScript?

Functions are treated like any other value: they can be assigned to variables, passed as arguments, returned from other functions, stored in data structures, and have a type (typeof is "function").

Q2: What is the difference between a function statement and a function expression?

A function statement (declaration) is hoisted with its full body and can be called before the line. A function expression is assigned to a variable, which is hoisted (undefined with var, TDZ with let/const) but the function body is only assigned at the line.

Q3: What is an anonymous function?

A function without a name. It is often assigned to a variable (const greet = function() {}) or used as a callback. The variable has a name, but the function itself does not.

Q4: What is a named function expression?

const greet = function sayHi(name) { return `Hello, ${name}`; };
``
`sayHi` is only visible inside the function. It is useful for recursion and produces better stack traces.

### Q5: Can you pass a function as an argument? Give an example.

```js
[1, 2, 3].map((n) => n * 2); // [2, 4, 6]
setTimeout(() => console.log("hi"), 1000);
button.addEventListener("click", () => console.log("clicked"));
``
### Q6: Can a function return another function? Give an example.

```js
function multiplier(factor) {
  return (n) => n * factor;
}
const double = multiplier(2);
double(5); // 10
``
### Q7: What is the difference between parameters and arguments?

Parameters are the variable names in the function definition. Arguments are the actual values passed when calling the function.

### Q8: What is a higher-order function?

A function that takes a function as an argument, returns a function, or both. Examples: `map`, `filter`, `reduce`, `compose`, `curry`, `setTimeout`.

### Q9: Do arrow functions have their own `this` and `arguments`?

No. Arrow functions inherit `this` from their lexical scope. They do not have their own `arguments` object (reference the enclosing function's). Use rest parameters (`...args`) in arrows.

### Q10: What is `typeof` for a function?

`"function"`. Functions are a distinct type, but under the hood they are callable objects.

### The Takeaway

First-class function interview questions test: the definition (functions as values), function statements vs expressions (hoisting), anonymous vs named functions, passing and returning functions, parameters vs arguments, higher-order functions, arrow function differences, and `typeof` for functions.