Difference between JavaScript Function Declaration and JavaScript Expression

Last Updated : 15 Feb 2025

JavaScript functions enable us to perform a variety of tasks, make crucial choices, perform computations, and even enhance the interactivity of our websites. We shall discover the distinction between "function expression" and "function declaration" in this post. Both use the term function, which is where they are comparable. The main distinction is that the function declaration has a function name, while the latter does not.

Declaration of Function

A function is declared using a function keyword in a function declaration, also referred to as a function statement.
A function name is required in the function declaration.
Functions are stand-alone structures that are not grouped inside functional blocks, so function declarations do not require variable assignments.
They run ahead of all other codes.
The function in the function declaration is accessible Both before and after the function definition.

Syntax

The following syntax is used for function declaration.

 
functionName(parameter1, prameter2){
//write functions code
}

Example:

The following example shows the javascript declaration for the arithmetic operation.

 
<!DOCTYPE html>
<html>
<head>
<title> Function expression in javascript </title>
</head>
<body style = "background-color:beige;">
<h2> difference between javascript Function expression and javascript function declaration </h2>
<h4> JavaScript Anonymous Function declaration </h4>
<p> We can create and declare the function without the function name. We can use the let variable for the function expression. </p>
<b id = "data_view"> </b>
<br>
<b id = "data_view1"> </b>
<script>
//create the variable with the 'let' keyword for a function expression
function cal_Substraction (x, y) {
let c = x - y;
return c;
}
document.getElementById("data_view").innerHTML = "Subtraction value: " + cal_Substraction(10, 4);
function cal_add(x, y) {
let c = x + y;
return c;
}
document.getElementById("data_view1").innerHTML = "Addition value: " + cal_add(10, 4);
</script>
</body>
</html>

Output:

The output shows the function declaration and its operation in an anonymous way.

Difference between JavaScript Function Declaration and JavaScript Expression

Function Expression

Without the function name, a function expression is comparable to a function declaration.
Variable assignments allow for the storage of function expressions.
Function expressions load and run once the program interpreter gets to the code line.
Only after the function definition is the function in the function expression accessible.

Syntax:

The following syntax is used for a function expression.

 
Var function_variable = function(parameter1, prameter2){
//write functions code
}

Example:

The following example shows the javascript expression.

 
<!DOCTYPE html>
<html>
<head>
<title> Function expression in javascript </title>
</head>
<body style = "background-color:beige;">
<h2> difference between javascript Function expression and javascript function declaration </h2>
<h4> JavaScript Anonymous Function Expression </h4>
<p> We can create and declare the function without the function name. We can use the let variable for the function expression. </p>
<b id = "data_view"> </b>
<br>
<b id = "data_view1"> </b>
<script>
//create the variable with the 'let' keyword for a function expression
let cal_Substraction = function (x, y) {
let c = x - y;
return c;
}
document.getElementById("data_view").innerHTML = "Subtraction value: " + cal_Substraction(10, 4);
let cal_add = function (x, y) {
let c = x + y;
return c;
}
document.getElementById("data_view1").innerHTML = "Addition value: " + cal_add(10, 4);
</script>
</body>
</html>

Output:

The output shows the function expression in an anonymous way and its operation.

Comparison between Function Declaration and Expression

The following table compares the function declaration and expression.

Function Declaration	Function Expression
A function declaration needs a function name for the coding.	A function expression works the same as a function declaration but doesn't need the function name.
The declaration of a function does not need the assignment of variables.	Variable assignments allow for the storage of function expressions.
They run ahead of all other codes.	Function expressions load and run once the program interpreter gets to the code line.
It is possible to access the function in the function declaration both before and after the function definition.	Only after the function definition is the function in the function expression accessible.
Declarations of functions are raised.	Expressions for functions are not lifted.
Syntax Function_name(parameter1, prameter2){ //write functions code }	Syntax Var function_variable = function(parameter1, prameter2){ //write functions code }

Conclusion

The Javascript function declaration and expression are important for the different functions and their operation. The developer and student use the function declaration and expression according to functionality and requirements.

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