What is the difference between `var functionName = function() {}` vs `function functionName() {}`?

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).

For example, a function expression:

// TypeError: functionOne is not a function
functionOne();

var functionOne = function() {
  console.log("Hello!");
};

And, a function declaration:

// Outputs: "Hello!"
functionTwo();

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

function declarations defined inside blocks were taken care of conflictingly between browsers. Strict mode (introduced in ES5) settled this by scoping function declarations to their enclosing block.