js中的try catch finally 中的return执行机制

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