Add additional error type-checking examples and use instanceof
in later examples
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1 changed files with 47 additions and 3 deletions
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@ -355,7 +355,9 @@ Of course, everything's possible! Programmers do make mistakes. Even in open-sou
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In our case, `try..catch` is meant to catch "incorrect data" errors. But by its nature, `catch` gets *all* errors from `try`. Here it gets an unexpected error, but still shows the same `"JSON Error"` message. That's wrong and also makes the code more difficult to debug.
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In our case, `try..catch` is meant to catch "incorrect data" errors. But by its nature, `catch` gets *all* errors from `try`. Here it gets an unexpected error, but still shows the same `"JSON Error"` message. That's wrong and also makes the code more difficult to debug.
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Fortunately, we can find out which error we get, for instance from its `name`:
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Fortunately, there are multiple ways to determine the type of error. Common methods are shown below.
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Using its `name` property:
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```js run
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```js run
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try {
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try {
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@ -367,6 +369,48 @@ try {
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}
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}
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```
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```
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Using its `constructor.name` property (read-only):
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```js run
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try {
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user = { /*...*/ };
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} catch(e) {
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*!*
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alert(e.constructor.name); // "ReferenceError" for accessing an undefined variable
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*/!*
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}
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```
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Comparing its `constructor` property to the specific error type:
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```js run
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try {
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user = { /*...*/ };
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} catch(e) {
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*!*
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if (e.constructor === RefferenceError) {
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alert('e is a ReferenceError'); // "ReferenceError" for accessing an undefined variable
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}
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*/!*
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}
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```
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Comparing its class type to another error type using the `instanceof` operator:
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```js run
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try {
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user = { /*...*/ };
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} catch(e) {
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*!*
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if (e instanceof RefferenceError) {
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alert('e is an instance of ReferenceError'); // "ReferenceError" for accessing an undefined variable
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}
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*/!*
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}
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```
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MDN suggests using the `constructor` property or `instanceof` operator in its [error examples](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error#Handling_a_specific_error).
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The rule is simple:
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The rule is simple:
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**Catch should only process errors that it knows and "rethrow" all others.**
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**Catch should only process errors that it knows and "rethrow" all others.**
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@ -398,7 +442,7 @@ try {
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} catch(e) {
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} catch(e) {
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*!*
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*!*
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if (e.name == "SyntaxError") {
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if (e instanceof SyntaxError) {
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alert( "JSON Error: " + e.message );
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alert( "JSON Error: " + e.message );
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} else {
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} else {
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throw e; // rethrow (*)
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throw e; // rethrow (*)
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@ -425,7 +469,7 @@ function readData() {
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*/!*
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*/!*
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} catch (e) {
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} catch (e) {
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// ...
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// ...
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if (e.name != 'SyntaxError') {
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if (!(e instanceof SyntaxError)) {
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*!*
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*!*
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throw e; // rethrow (don't know how to deal with it)
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throw e; // rethrow (don't know how to deal with it)
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*/!*
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*/!*
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