Javascript函数声明和函数表达式
deltamaster
posted @ Mar 21, 2014 09:00:02 PM
in JavaScript
, 4015 阅读
先看一下下面的代码：
var a = function(x) {document.write(x + "\n");}(1); // OK - output "1"
function(x) {document.write(x + "\n");}(1); // Uncaught SyntaxError: Unexpected token (
小伙伴们就议论开了，为什么第一行的程序运行成功，而第二行的程序有语法错误呢？错误信息里提到的“(”是指哪一个括号呢？
之后又诞生了一系列的写法，来分别验证结果：
function f(x){document.write(x + "\n");}(1); // No output
(function(x) {document.write(x + "\n");})(1); // OK - output "1"
(function(x) {document.write(x + "\n");}(1)); // OK - output "1"
+function(x) {document.write(x + "\n");}(1); // OK - output "1"
~function(x) {document.write(x + "\n");}(1); // OK - output "1"
在这里先直接揭晓答案，在语句开头出现function，则被视为函数声明（FunctionDeclaration），否则视为函数表达式（FunctionExpression）。两者都同样构造一个函数对象，但是有两个区别：
- 函数声明必须有函数名作为函数标识符，而对于函数表达式则是可选的。（上面报出语法错误的那行代码，实际上是说不允许函数声明不含有标识符）
- 函数声明是与语句（Statement）平级，不求值的语法元素，而表达式有返回值。（上面没有输出的那行代码，是因为被视为一句函数声明，还有一句是(1);语句）
为何这样设计？
当解释器进行语法分析时，期待读入一个Statement或FunctionDeclaration，此时读入到词法元素“function”。此时就有了二义性，这里既可能是FunctionDeclaration的第一个词法元素，也可能是ExpressionStatement的第一个语法元素（即其中FunctionExpression的第一个词法元素），为了避免这种二义性，规定在这种情况下，将这个“function”视作FunctionDeclaration的第一个词法元素，于是就有了上面的结论和实验结果。
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