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Table of Contents
2. Hoisting Behavior
3. When Are They Created?
Summary of Key Differences
Home Web Front-end Front-end Q&A What is the difference between function declaration and function expression in JavaScript?

What is the difference between function declaration and function expression in JavaScript?

Aug 01, 2025 am 07:17 AM

Function declarations are directly defined using the function keyword, and function expressions assign functions to variables; 2. Function declarations are completely promoted and can be called in advance, function expressions are only promoted variables and cannot be called in advance; 3. Function declarations are created during parsing, function expressions are created at runtime, and conditional definitions are supported; 4. Function declarations are suitable for scenarios that require advance call and readability, and function expressions are suitable for dynamic assignment, callbacks and conditional logic scenarios. Therefore, function declarations are available earlier and function expressions are executed sequentially.

What is the difference between function declaration and function expression in JavaScript?

The main difference between a function declaration and a function expression in JavaScript lies in syntax , hoisting behavior , and when they are created during execution.

What is the difference between function declaration and function expression in JavaScript?

1. Syntax and Structure

  • Function Declaration
    Defined with the function keyword followed by a name, parentstheses, and a block of code.

     function greet() {
      console.log("Hello!");
    }
  • Function Expression
    A function that is assigned to a variable. It can be named or anonymous.

    What is the difference between function declaration and function expression in JavaScript?
     const greet = function() {
      console.log("Hello!");
    };

    Or with a name (useful for debugging):

     const greet = function greetFunc() {
      console.log("Hello!");
    };

2. Hoisting Behavior

This is where the key practical difference shows up.

What is the difference between function declaration and function expression in JavaScript?
  • Function declarations are hoisted completely.
    That means you can call them before they are defined in the code.

     greet(); // ? Works fine
    function greet() {
      console.log("Hello!");
    }
  • Function expressions are not fully hoisted.
    Only the variable declaration is hoisted (if using var ), but not the function assignment. So calling it before assignment causes an error.

     saysHello(); // ? TypeError: sayHello is not a function
    
    const saysHello = function() {
      console.log("Hello!");
    };

    Note: With const or let , the variable is in the "temporal dead zone" until assigned, so accessing it early throws an error.


3. When Are They Created?

  • Function declarations are created at parse time , before any code is executed.
  • Function expressions are created at runtime , when the code reaches that line.

This means function expressions can be conditionally defined:

 let performAction;

if (true) {
  performAction = function() {
    console.log("Doing task A");
  };
} else {
  performAction = function() {
    console.log("Doing task B");
  };
}

You generally can't conditionally declare functions with declarations (it works in some environments but is unreliable and not recommended).


4. Use Cases

  • Use function declarations for utility functions you know you'll need and want to keep code readable.
  • Use function expressions when:
    • Assigning functions to object properties (methods),
    • Passing functions as arguments,
    • Creating IIFEs (Immediately Invoked Function Expressions),
    • Or needing conditional logic in function creation.

Summary of Key Differences

Feature Function Declaration Function Expression
Syntax function name() {} const name = function() {}
Hoisting Fully hoisted Not hoisted (only var declared)
Can be called before def? ? Yes ? No (with const / let )
Conditional definition? ? Not reliable ? Yes

So, the choice depends on your needs: readability and hoisting (use declaration), or flexibility and control (use expression).
Basically, just remember: declarations are ready early, expressions wait their turn .

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