var functionName = function() {} vs function functionName() {}

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The difference is that functionOne is a function expression and so only defined when that line is reached, whereas functionTwo is a function declaration and is defined as soon as its surrounding function or script is executed (due to hoisting).

Expression

// TypeError: functionOne is not a function
functionOne();
 
var functionOne = function() {
  console.log("Hello!");
};

Declaration

// Outputs: "Hello!"
functionTwo();
 
function functionTwo() {
  console.log("Hello!");
}

Aliasing Functions

Functions can be aliased by assigning one function to multiple variables (e.g., var xyz = abc). This makes xyz and abc references to the same function object.

Naming Functions

A function’s name attribute depends on how it’s defined. Named functions like function abc(){} have their name property set automatically, while anonymous functions assigned to variables have an empty name (e.g., var abc = function(){}).