JS Errors - Throw and Try to Catch

1. Statement Overview

2. try/catch/finally

• finally must be executed
• if returned in catch and finally block in the same time, returned by finally comes first.

var myFunc = function(){
    try{
        console.log('myFunc: exec try');
        throw new Error();
    } catch(e) {
        console.log('myFunc: exec catch');
        return "Catch";
    } finally {
        console.log('myFunc: exec finally');
        return "Finally";
    }
    console.log('myFunc: to be ended.');
    return "Done";
};

var x = myFunc();
console.log('x=', x);


//********* result ***********
//myFunc: exec try
//myFunc: exec catch
//myFunc: exec finally
//x= Finally

var myFunc = function(){
    try{
        console.log('myFunc: exec try');
        throw new Error();
    } catch(e) {
        console.log('myFunc: exec catch');
        return "Catch";
    } finally {
        console.log('myFunc: exec finally');
        //return "Finally";
    }
    console.log('myFunc: to be ended.');
    return "Done";
};

var x = myFunc();
console.log('x=', x);

//********* result ***********
//myFunc: exec try
//myFunc: exec catch
//myFunc: exec finally
//x= Catch

3. Throws

mynote: 只要是 throw，無論 throw 的是 Error 物件或普通變數，都會觸發 catch (The exception can be a JavaScript String, a Number, a Boolean or an Object:)

// trail code template
try{
  throw "Too big";          // throw a text
  //throw 500;              // throw a number
  //throw new Error();      // throw an Error Object
  //throw new Error('my error msg');
  //throw new Exception();  // Not an defined object
}catch(e){
  console.log(e);
  console.log(e.toString());
  console.log(typeof e);
  console.log(e.constructor);
  console.log(e.name);
  console.log(e.message);
}

// case 1: throw "Too big"
console.log(e);             // "Too big"
console.log(e.toString());  // "Too big"
console.log(typeof e);      // "string"
console.log(e.constructor); // ƒ String() { [native code] }
console.log(e.name);        // undefined
console.log(e.message);     // undefined

// case 2: throw 500
console.log(e);             // 500
console.log(e.toString());  // "500"
console.log(typeof e);      // "number"
console.log(e.constructor); // ƒ Number() { [native code] }
console.log(e.name);        // undefined
console.log(e.message);     // undefined

// case 3: throw new Error()
console.log(e);             // Error
                            //    at <anonymous>:4:8
                            //    at run_code (default-source2.js?v=a1:467)
                            //    at HTMLAnchorElement.onclick ((index):219)
console.log(e.toString());  // "Error"
console.log(typeof e);      // "object"
console.log(e.constructor); // ƒ Error() { [native code] }
console.log(e.name);        // "Error"
console.log(e.message);     // ""

// case 4: throw new Error('my error msg')
console.log(e);             // Error: my error msg
                            //    at <anonymous>:4:8
                            //    at run_code (default-source2.js?v=a1:467)
                            //    at HTMLAnchorElement.onclick ((index):219)
console.log(e.toString());  // "Error: my error msg"
console.log(typeof e);      // "object"
console.log(e.constructor); // ƒ Error() { [native code] }
console.log(e.name);        // "Error"
console.log(e.message);     // "my error msg"

// case 5: throw new Exception()
console.log(e);             // ReferenceError: Exception is not defined
                            //    at <anonymous>:4:2
                            //    at run_code (default-source2.js?v=a1:467)
                            //    at HTMLAnchorElement.onclick ((index):219)
console.log(e.toString());  // "ReferenceError: Exception is not defined"
console.log(typeof e);      // "object"
console.log(e.constructor); // ƒ ReferenceError() { [native code] }
console.log(e.name);        // "ReferenceError"
console.log(e.message);     // "Exception is not defined"

4. Error Objects

JavaScript will actually create an Error object with two properties: name and message.

Six different values can be returned by the error name property:

4.1. 1. EvalError

Newer versions of JavaScript does not throw any EvalError. Use SyntaxError instead.

4.2. 2. RangeError

e.g. num.toPrecision(500);

4.3. 3. Reference Error

e.g. A variable that has not been declared:

4.4. 4. Syntax Error

e.g. eval("alert('Hello)");

4.5. 5. Type Error

If you use a value that is outside the range of expected types:

e.g. num.toUpperCase();

4.6. 6. URI Error

Use illegal characters in a URI function

e.g. decodeURI("%%%");

5. Non-Standard Error Object Properties

Mozilla and Microsoft defines some non-standard error object properties.

Do not use these properties in public web sites. They will not work in all browsers.