錯誤處理和控制流分支
回撥通常用於提供錯誤處理。這是控制流分支的一種形式,其中一些指令僅在發生錯誤時執行:
const expected = true;
function compare(actual, success, failure) {
if (actual === expected) {
success();
} else {
failure();
}
}
function onSuccess() {
console.log('Value was expected');
}
function onFailure() {
console.log('Value was unexpected/exceptional');
}
compare(true, onSuccess, onFailure);
compare(false, onSuccess, onFailure);
// Outputs:
// "Value was expected"
// "Value was unexpected/exceptional"
上面 compare()
中的程式碼執行有兩個可能的分支:success
當預期值和實際值相同時,error
當它們不同時。當控制流應該在一些非同步指令之後分支時,這尤其有用:
function compareAsync(actual, success, failure) {
setTimeout(function () {
compare(actual, success, failure)
}, 1000);
}
compareAsync(true, onSuccess, onFailure);
compareAsync(false, onSuccess, onFailure);
console.log('Doing something else');
// Outputs:
// "Doing something else"
// "Value was expected"
// "Value was unexpected/exceptional"
應該注意,多個回撥不必相互排斥 - 可以呼叫這兩種方法。類似地,compare()
可以用可選的回撥編寫(通過使用 noop 作為預設值 - 參見 Null Object 模式 )。