JavaScript 函数声明 VS 函数表达式

Function Declaration :
function Identifier ( FormalParameterList opt ){ FunctionBody }

Function Expression :
function Identifier opt ( FormalParameterList opt ){ FunctionBody }

If a function foo(){} is part of, say, assignment expression, it is considered a function expression. If, on the other hand, function foo(){} is contained in a function body or in a (top level of) program itself — it is parsed as a function declaration.

A somewhat less obvious example of function expression is the one where function is wrapped with parenthesis — (function foo(){}). 

The reason it is an expression is again due to a context: "(" and ")" constitute a grouping operator and grouping operator can only contain an expression.