In JavaScript, functions can be defined in various ways. One method is using function expressions. A function expression lets you define a function and assign it to a variable, which can then be used as a function.

The syntax for a function expression is:

var functionName = function(parameters){
// function body
};

The syntax above defines a functionName variable that holds a function expression. The function can have one or more parameters in its parentheses. The function body contains the code to execute when calling the function.

Here's an example of a function expression:

var add = function(num1, num2){
return num1 + num2;
};
 
console.log(add(2, 3)); // output: 5

The function expression add takes two numbers, num1 and num2, and returns their sum.

Anonymous functions are one benefit of function expressions. Anonymous functions lack a name. They are useful when passing a function as an argument to another function or using it as a callback function.

Here is an anonymous function expression example:

 
var multiply = function(num1, num2){
  return num1 * num2;
};
 
console.log(multiply(2, 3)); // output: 6
 

In the above example, the function expression multiply is anonymous because it does not have a name.

Function expressions can also be used as IIFE (Immediately Invoked Function Expression) which is a function that is executed right after it's defined. This pattern is useful when you want to create a private scope for your code, and avoid polluting the global namespace.

Here is an example of an IIFE using function expression:

(function(){
var message = "Hello, World!";
console.log(message);
})();

In the above example, we define an anonymous function and immediately invoke it. The function declares a variable message and logs it to the console. Since the variable message is defined inside the function, it cannot be accessed outside of it.