JavaScript function expression vs function declaration

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在Javascript中创建function有两种方式：function expression 和function declaration。它们的定义分别是：

FunctionDeclaration:

FunctionIdentifier(formatParameterListOpt) { FunctionBody}

FunctionExpression:

Function IdentifierOpt(formatParameterListOpt){FunctionBody}

从它们的定义来看，唯一的区别是identifier是否是optional。如果identifier出现了，我们该如何区分它们呢？ECMAScript通过如下的方式来区分它们，如果function foo(){}是一个赋值表达式，此时是function expression。而functionfoo(){}在一个函数体内或者在程序中，此时是function declaration。

function foo(){} // declaration, since it is part of a programvar bar=function foo(){};// expression, since it is part of an assignmentExpressionnew function bar(){}; // expression, since it is part of NewExpression(function() {      function bar(){}  // declaration, since it is part of FunctionBody})();

有一种特殊的情况是：当函数被小括号（grouping operator）包装的时候，此时作为function expression

function foo(){} // function declaration(function foo(){}); // function expression, due to grouping operator

当然，function declaration与function expression还是有区别的。

1. functiondeclaration hoisting: 函数declaration在其它表达式之前被解释以及执行。

alert(fn());  // output: hello JavaScriptfunction fn() {return'hello JavaScript';}

2. functiondeclaration在block { …}中，尤其是if,while,for表达式里的行为是未定义的，never do it.

//never do it// some browser will declare `foo`  as the one returning ‘first’// while others returning ‘second’if (true){function foo(){return ‘first’;}}else{function foo(){return ‘second’;}}// Instead, use function expressionvar foo;if (true){foo = function(){return ‘first’;};}else {foo = function() {return ‘second’;}}

当function declaration有名字（即identifier）,曰named function expression，如var bar = function foo(){}; foo是函数名，需要注意的是foo的scope

var f = function foo(){return typeof foo; // foo is available in this inner scope}// foo is never visible outsidetypeof foo; // undefinedf(); // function

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