JavaScript 匿名函数有哪几种执行方式?

Question

JavaScript 匿名函数有哪几种执行方式

Answer 1

我们从规范出发，看看EcmaScript的上下文无关文法。参考《ECMA-262, 3rd edition, December 1999》。ES3简单一些，ES6同理。Program :SourceElementsSouceElements :SouceElementSouceElements SourceElementSouceElement :StatementFunctionDeclarationStatement :BlockVariableStatementEmptyStatementExpressionStatementIfStatementIterationStatementContinueStatementBreakStatementReturnStatementWithStatementLabelledStatementSwitchStatementThrowStatementTryStatementExpressionStatement :[lookahead ∉ {{, function}] Expression ;FunctionDeclaration :function Identifier ( FormalParameterList[opt] ) { FunctionBody }这几个产生式表明了以Program为根的语法树结构。- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -（1）首先，“function(){}()”无法归约为CallExpression。CallExpression<- LeftHandSideExpression<- PostfixExpression<- UnaryExpression<- MultiplicativeExpression<- AdditiveExpression<- ShiftExpression<- RelationalExpression<- EqualityExpression<- BitwiseANDExpression<- BitwiseXORExpression<- BitwiseORExpression<- LogicalANDExpression<- LogicalORExpression<- ConditionalExpression<- AssignmentExpression<- Expression[失败了] <- ExpressionStatement<- Statement<- SourceElement<- SourceElements<- Program失败的原因是，Expression要想归约为ExpressionStatement，就不能以左大括号“{”或“function”开头。（2）那么，我们只能认为“function(){}()”是由两部分构成的了。program-> SourceElements-> SourceElements SourceElement-> SourceElement SourceElement把“funtion(){}”和“()”都分别向SourceElement归约。然而，这又不行。（3）“funtion(){}”无法归约为SourceElement。因为，无论是先归约为Statement，还先归约为FunctionDeclaration都不行。归约为Statement要求不能以function开头，（与上面的失败原因相同）归约为FunctionDeclaration要求必须指定函数名。（4）然而，我们又没有其他归约方法了，只能失败了。- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -而加上符号以后，例如“!function(){}()”，并不是以“function开头”的，所以通过了Expression到ExpressionStatement的归约。!function(){}()<- ! function Identifier[opt] ( FormalParameterList[opt] ) { FunctionBody } Arguments<- ! FunctionExpression Arguments<- ! MemberExpression Arguments<- ! CallExpression<- ! LeftHandSideExpression<- ! PostfixExpression<- ! UnaryExpression<- UnaryExpression<- MultiplicativeExpression<- AdditiveExpression<- ShiftExpression<- RelationalExpression<- EqualityExpression<- BitwiseANDExpression<- BitwiseXORExpression<- BitwiseORExpression<- LogicalANDExpression<- LogicalORExpression<- ConditionalExpression<- AssignmentExpression<- Expression<- ExpressionStatement<- Statement<- SourceElement<- SourceElements<- Program同理可以推导(function(){}())，(function(){})()- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -值得指出的是，“function(){}()”就算指定了函数名，也不行。因为后面的“()”同样不能归约为SourceElement。而“function f(){}(1)”就是合法的，“function f(){}”归约为FunctionDeclaration，而“(1)”可以归约为Expression，视为括号运算符，或称为分组运算符。(Expression)<- PrimaryExpression<- MemberExpression<- NewExpression<- LeftHandSideExpression<- PostfixExpression<- UnaryExpression<- MultiplicativeExpression<- AdditiveExpression<- ShiftExpression<- RelationalExpression<- EqualityExpression<- BitwiseANDExpression<- BitwiseXORExpression<- BitwiseORExpression<- LogicalANDExpression<- LogicalORExpression<- ConditionalExpression<- AssignmentExpression<- Expression