js中的try catch finally 中的return执行机制

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js中的try catch finally 中的return执行机制

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try catch finally 子句中结合 return 会有怎样的效果？

try catch finally 子句是 ECMA2015 - try catch statment 中定义的语法。其执行机制如下

try {Block} catch 语句 Let B be the result of evaluating Block. If B.[[type]] is throw, then Let C be CatchClauseEvaluation of Catch with parameter B.[[value]]. Else B.[[type]] is not throw, Let C be B. If C.[[type]] is return, or C.[[type]] is throw, return Completion(C). If C.[[value]] is not empty, return Completion(C). Return Completion{[[type]]: C.[[type]], [[value]]: undefined, [[target]]: C.[[target]]}.

try catch 语句首先会执行try包裹的代码块，根据代码块执行结果判断是否进入catch的代码块，如果catch的代码块有返回结果，则抛出异常时会覆盖try语句的执行结果。否则因为Block是异常抛出，返回undefined。

function test (){ try { return 0 } catch (e) { return 1 } } test() // 0 function test1 (){ try { throw new Error() return 0 // 这里不会执行到 } catch (e) { return 1 } } test1() //1 try {Block} finally 语句 Let B be the result of evaluating Block. Let F be the result of evaluating Finally. If F.[[type]] is normal, let F be B. If F.[[type]] is return, or F.[[type]] is throw, return Completion(F). If F.[[value]] is not empty, return Completion(F). Return Completion{[[type]]: F.[[type]], [[value]]: undefined, [[target]]: F.[[target]]}.

Block 和 finally包裹的代码会依次执行，同时记录其返回结果 B 与 F，如果F 有返回值(正常或者异常返回)，则返回F的结果，否则返回B的结果

function test (){ try { throw new Error() } finally { return 1 } } test() // 1 function test1 (){ try { throw new Error() } finally { throw new Error(); } } test1() // Uncaught Error function test2 (){ try { return 0 } finally { } } test2() // 0 function test3 (){ try { return 0 } finally { return 1 } } test3() // 1 try {Block} catch finally 语句 Let B be the result of evaluating Block. If B.[[type]] is throw, thenLet C be CatchClauseEvaluation of Catch with parameter B.[[value]]. Else B.[[type]] is not throw, let C be B. Let F be the result of evaluating Finally. If F.[[type]] is normal, let F be C. If F.[[type]] is return, or F.[[type]] is throw, return Completion(F). If F.[[value]] is not empty, return NormalCompletion(F.[[value]]). Return Completion{[[type]]: F.[[type]], [[value]]: undefined, [[target]]: F.[[target]]}.

相当于 try catch 和try finally 的组合执行，先执行try catch 的处理结果再将其结果与finally合并。

一些有意思的案例 function test() { var a = 1; try { return a; } finally { ++a; } } test() // 1 function test1() { var a = 1; try { return a; } finally { ++a; return a; } } test1() // 2 function test2() { var obj = {a:1}; try { return obj; } finally { ++obj.a; } } test2().a // 2 function test3() { var obj = {a:1}; try { return obj.a; } finally { ++obj.a; } } test1() // 1

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js中的try catch finally 中的return执行机制

js中的try catch finally 中的return执行机制

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