Function Expression

A function expression is when a function is assigned to a variable.
This is very similar to a function declaration except the name can be omitted to create an anonymous function.
A function expression is run as soon as it is defined.
Function expressions are not hoisted.
You do have to include a semicolon at the end.

var IsEven = function (num) { 
   return num % 2 === 0;

Named Function Expression

This function cannot be invoked using its name. You must use the variable name.
If you have assigned it to a variable you cannot call the function name
The name of the function only exists inside the function scope, it is not accessbile outside.
This is called a Function Literal

var myVar = function add () { 
   return arguments[0] + arguments[1];

Anonymous Function Expression

The function name can be omitted but it is best practice to always include a name so it appears on the call stack when debugging.

var myVar = function() { 
   return arguments[0] + arguments[1];

This is a named function expression

var checkNum = { 
'isEven' : function IsEven(num) {
      return num % 2 === 0;
console.log(checkNum.IsEven(2) ); // true

The variable the function expression is assigned to will have a 'name' property

var aa = function() {} 
console.log(; // aa

The name will not change if it is assigned to another variable

var bb = aa; 
console.log(; // aa

If the function name is present then the name will be the function name.
If the function name is anonmous the variable name will be used.

var cc = function dd() {}; 
console.log(; // dd

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