How can we catch errors in our programs in JavaScript
- try-catch block
- catch method
try-catch block
Anything inside of try block, if there is any errors then handle them in catch block. Catch block has a parameter about error.
function a() {
try {
let num = 1;
let num2 = 2;
novalue;
return num + num2;
} catch (error) {
throw error;
}
}
a(); throw error
^
ReferenceError: novalue is not defined
at a (/Users/jayjo/Desktop/j.js:23:5)
at Object.<anonymous> (/Users/jayjo/Desktop/j.js:29:1)
at Module._compile (internal/modules/cjs/loader.js:1137:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1157:10)
at Module.load (internal/modules/cjs/loader.js:985:32)
at Function.Module._load (internal/modules/cjs/loader.js:878:14)
at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
at internal/main/run_main_module.js:17:47With Finally block
function a() {
try {
let num = 1;
let num2 = 2;
novalue;
return num + num2;
} catch (error) {
throw error;
} finally {
console.log("It will finally run.");
}
}
a();
It will finally run.
/Users/jayjo/Desktop/j.js:26
throw error;
^
ReferenceError: novalue is not defined
at a (/Users/jayjo/Desktop/j.js:23:5)
at Object.<anonymous> (/Users/jayjo/Desktop/j.js:31:1)
at Module._compile (internal/modules/cjs/loader.js:1137:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1157:10)
at Module.load (internal/modules/cjs/loader.js:985:32)
at Function.Module._load (internal/modules/cjs/loader.js:878:14)
at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
at internal/main/run_main_module.js:17:47Even if there is an error in try block, the finally block will be executed.
Nested try-catch block
function a() {
try {
try {
let num = 1;
let num2 = 2;
novalue;
return num + num2;
} catch (error) {
throw new Error(error);
}
} catch (error) {
console.log(error);
}
}
a();Error: ReferenceError: novalue is not defined
at a (/Users/jayjo/Desktop/j.js:27:13)
at Object.<anonymous> (/Users/jayjo/Desktop/j.js:33:1)
at Module._compile (internal/modules/cjs/loader.js:1137:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1157:10)
at Module.load (internal/modules/cjs/loader.js:985:32)
at Function.Module._load (internal/modules/cjs/loader.js:878:14)
at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
at internal/main/run_main_module.js:17:47These types of try catch blocks can be used to catch synchronous errors.
However, what if there is asynchronous code in try block?
function a() {
try {
setTimeout(function () {
noValue;
}, 10000);
return true;
} catch (error) {
console.log(error);
}
}
console.log(a());true
/Users/jayjo/Desktop/j.js:22
noValue;
^
ReferenceError: noValue is not defined
at Timeout._onTimeout (/Users/jayjo/Desktop/j.js:22:7)
at listOnTimeout (internal/timers.js:549:17)
at processTimers (internal/timers.js:492:7)During 10 seconds, catch block can not catch the error. Thus, we have Async Error Handling.