Expression vs Declaration

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If an expression starts with the keyword function, it is a function expression.

var dayName = function() { var names = ["Sunday", "Monday", "Tuesday", "Wednesday",

"Thursday", "Friday", "Saturday"];

return function(number) {

return names[number];

};

}();

However, if a statement starts with function, it is a function declaration,

var names = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];

function dayName(number) {

return names[number];

}

which requires a name and, not being an expression, cannot be called by writing parentheses after it.

You can think of the extra wrapping parentheses as a trick to force the function to be interpreted as an expression.