What is the difference between a function declaration and a function expression?

Direct answer

A function declaration defines a named function as a declaration and is initialized before normal execution, while a function expression creates a function as part of an expression and follows the initialization rules of its containing variable.

A declaration looks like function greet() {}. An expression can look like const greet = function () {}; or const greet = () => {};.

Function declarations can usually be called earlier in the same scope because the function binding is initialized during environment setup.
A function expression assigned to let or const cannot be used before that variable is initialized.
Function expressions are convenient when functions are passed around as values, assigned conditionally, or created inline.