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What is the difference between function declarations and function expressions?
What is the difference between function declarations and function expressions?
Jul 15, 2025 am 02:28 AM
Function declarations and function expressions differ in hoisting and runtime behavior. 1. The function declaration will be completely promoted and can be called before definition; 2. The function expression only has the variable declaration, and the assignment is not promoted, and an error will be reported when calling in advance; 3. The name of the named function expression is only valid inside the function; 4. When using function declaration, it is suitable for creating reusable functions that need to be called in advance. Function expressions are suitable for conditional assignment, argument transmission or creation of anonymous functions; 5. The arrow function is the abbreviation of function expressions, and there is no declaration form.
Function declarations and function expressions are both ways to define functions in JavaScript, but they behave differently, especially when it comes to hoisting and how they're evaluated at runtime.
Function Declarations Are Hoisted
A function declaration is straightforward:
function saysHello() {
console.log('Hello');
}This type of function is hoisted , which means you can call it before it's defined in the code. For example:
saysHello(); // This works!
function saysHello() {
console.log('Hello');
}JavaScript moves the entire function declaration to the top of the current scope during the compilation phase, so the above works without errors.
Function Expressions Are Not Hoisted
A function expression looks like this:
const saysGoodbye = function() {
console.log('Goodbye');
}; Here, the function is assigned to a variable. Since only the variable declaration (not its assignment) is hoisted, trying to call sayGoodbye() before it's assigned will result in an error:
sayGoodbye(); // TypeError: sayGoodbye is not a function
const saysGoodbye = function() {
console.log('Goodbye');
}; In this case, sayGoodbye exists due to hoisting, but it's still undefined when we try to call it because the assignment hasn't happened yet.
Named vs. Anonymous Function Expressions
You can also give a name to a function expression, which is useful for debugging:
const greet = function saysHi() {
console.log('Hi!');
};
greet(); // Works fine
saysHi(); // ReferenceError: sayHi is not defined Even though the function has a name ( sayHi ), that name is only accessible inside the function itself — not outside. So from the outside, you still refer to it by the variable name ( greet ).
When to Use Which?
- Use function declarations when you want to create reusable functions that can be called anywhere in your scope, even before their definition.
- Use function expressions when:
- You need to assign a function conditionally
- Pass a function as an argument to another function
- Create anonymous or named inline functions
Also, with ES6 arrow functions, function expressions became more concise:
const multiply = (a, b) => a * b;
Arrow functions are always function expressions — there's no such thing as an arrow function declaration.
So depending on whether or not you need hoisting and how you plan to use the function, you'll choose between a declaration and an expression.
Basically that's it.
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