流程控制与错误处理

while (x < 10) {
  x++;
}

var x = 1;
{
  var x = 2;
}
alert(x); // 输出的结果为 2

if (condition) {
  statement_1;
} else {
  statement_2;
} //推荐使用严格的语句块模式，语句 else 可选

if (condition_1) {
  statement_1;
} else if (condition_2) {
  statement_2;
} else if (condition_n_1) {
  statement_n;
} else {
  statement_last;
}

if (条件) {
  当条件为真的时候，执行语句 1;
  当条件为真的时候，执行语句 2;
} else {
  当条件为假的时候，执行语句 3;
  当条件为假的时候，执行语句 4;
}

if ((x = y)) {
  /* statements here */
}

var b = new Boolean(false);
if (b) //结果视为真
if (b == true) // 结果视为假

function checkData() {
  if (document.form1.threeChar.value.length == 3) {
    return true;
  } else {
    alert(
      "Enter exactly three characters. " +
        document.form1.threeChar.value +
        " is not valid.",
    );
    return false;
  }
}

switch (expression) {
   case label_1:
      statements_1
      [break;]
   case label_2:
      statements_2
      [break;]
   ...
   default:
      statements_def
      [break;]
}

switch (fruittype) {
  case "Oranges":
    document.write("Oranges are $0.59 a pound.<br>");
    break;
  case "Apples":
    document.write("Apples are $0.32 a pound.<br>");
    break;
  case "Bananas":
    document.write("Bananas are $0.48 a pound.<br>");
    break;
  case "Cherries":
    document.write("Cherries are $3.00 a pound.<br>");
    break;
  case "Mangoes":
  case "Papayas":
    document.write("Mangoes and papayas are $2.79 a pound.<br>");
    break;
  default:
    document.write("Sorry, we are out of " + fruittype + ".<br>");
}
document.write("Is there anything else you'd like?<br>");

throw expression;

throw "Error2"; // String type
throw 42; // Number type
throw true; // Boolean type
throw {
  toString: function () {
    return "I'm an object!";
  },
};

// Create an object type UserException
function UserException(message) {
  this.message = message;
  this.name = "UserException";
}

// Make the exception convert to a pretty string when used as
// a string (e.g. by the error console)
UserException.prototype.toString = function () {
  return this.name + ': "' + this.message + '"';
};

// Create an instance of the object type and throw it
throw new UserException("Value too high");

function getMonthName(mo) {
  mo = mo - 1; // Adjust month number for array index (1 = Jan, 12 = Dec)
  var months = [
    "Jan",
    "Feb",
    "Mar",
    "Apr",
    "May",
    "Jun",
    "Jul",
    "Aug",
    "Sep",
    "Oct",
    "Nov",
    "Dec",
  ];
  if (months[mo]) {
    return months[mo];
  } else {
    throw "InvalidMonthNo"; //throw keyword is used here
  }
}

try {
  // statements to try
  monthName = getMonthName(myMonth); // function could throw exception
} catch (e) {
  monthName = "unknown";
  logMyErrors(e); // pass exception object to error handler -> your own function
}

catch (catchID) {
  statements
}

try {
  throw "myException"; // generates an exception
} catch (e) {
  // statements to handle any exceptions
  logMyErrors(e); // pass exception object to error handler
}

openMyFile();
try {
  writeMyFile(theData); //This may throw a error
} catch (e) {
  handleError(e); // If we got a error we handle it
} finally {
  closeMyFile(); // always close the resource
}

function f() {
  try {
    console.log(0);
    throw "bogus";
  } catch (e) {
    console.log(1);
    return true; // this return statement is suspended
    // until finally block has completed
    console.log(2); // not reachable
  } finally {
    console.log(3);
    return false; // overwrites the previous "return"
    console.log(4); // not reachable
  }
  // "return false" is executed now
  console.log(5); // not reachable
}
f(); // console 0, 1, 3; returns false

function f() {
  try {
    throw "bogus";
  } catch (e) {
    console.log('caught inner "bogus"');
    throw e; // this throw statement is suspended until
    // finally block has completed
  } finally {
    return false; // overwrites the previous "throw"
  }
  // "return false" is executed now
}

try {
  f();
} catch (e) {
  // this is never reached because the throw inside
  // the catch is overwritten
  // by the return in finally
  console.log('caught outer "bogus"');
}

// OUTPUT
// caught inner "bogus"

function doSomethingErrorProne () {
  if (ourCodeMakesAMistake()) {
    throw (new Error('The message'));
  } else {
    doSomethingToGetAJavascriptError();
  }
}
....
try {
  doSomethingErrorProne();
}
catch (e) {
  console.log(e.name); // logs 'Error'
  console.log(e.message); // logs 'The message' or a JavaScript error message)
}