JavaScript 错误处理和控制流分支
示例
回调通常用于提供错误处理。这是控制流分支的一种形式，其中某些指令仅在发生错误时才执行：
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);
//输出：
// "Value was expected"
// "Value was unexpected/exceptional"compare()上面的代码执行有两个可能的分支：success期望值和实际值相同时以及error不同时。当控制流应在某些异步指令之后分支时，这特别有用：
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');
//输出：
// "Doing something else"
// "Value was expected"
// "Value was unexpected/exceptional"应该注意的是，多个回调不必是互斥的–可以调用这两种方法。类似地，compare()可以使用可选的回调（通过使用noop作为默认值-参见NullObject模式）来编写。