What happens when JavaScript variable name and function name is the same?

Functions are a type of object which are a type of value.

Values can be stored in variables (and properties, and passed as arguments to functions, etc).

A function declaration:

• Creates a named function
• Creates a variable in the current scope with the same name as the function (unless such a variable already exists)
• Assigns the function to that variable
• Is hoisted

A var statement:

• Creates a variable in the current scope with the specified name (unless such a variable already exists)
• Is hoisted
• Doesn't assign a value to that variable (unless combined with an assignment operator)

Both your declaration and var statement are hoisted. Only one of them assigns a value to the variable a.

In JavaScript both function declaration and variable declarations are hoisted to the top of the function, if defined in a function, or the top of the global context, if outside a function. And function declaration takes precedence over variable declarations (but not over variable assignment).

Function Declaration Overrides Variable Declaration When Hoisted

First you declare a variable:

var a; // value of a is undefined 

Second, the value of a is a function because function declaration takes precedence over variable declarations (but not over variable assignment):

function a(x) {
  return x * 2;
}

And that is what you get when you call alert(a);.

But, if instead of declaring a variable you make variable assignment: var a = 4; then the assigned value 4 will prevail.