https://www.youtube.com/watch?v=z0bFffFcUOA

callstack.js

// Call stack is whole tree of nested function calls

function A() {
  return B();
}

function B() {
  return C();
}

function C() {
  return D();
}

function D() {
  return "data";
}
// uncomment
// console.log(A());

// **************************************************************************************************
// Lets track how error propagates thru callstack:
function A() {
  try {
    return B();
  } catch (err) {
    console.log(err.stack);
  }
}

function B() {
  return C();
}

function C() {
  return D();
}

function D() {
  return error
}

console.log(A());

try_catch.js

// Working with try/catch for errors

try {
  console.log("Start of try runs");
  nonvariable; // error, variable is not defined!
  console.log("End of try (never reached)"); // try block interupted, so this never be executed
} catch (error) {
  console.log(`Error has occured!`);
}
console.log("...Then the execution continues");

// try..catch only works for runtime errors

// JS engine can't read this code, so cannot run
/* commented
try {
    &*()
} catch(e) {
// JS cannot catch syntax errors!
}
*/

// ***************************************************************************
// Default error object structure
try {
  nonvariable; // error, variable is not defined!
} catch (err) {
  console.log(err.name); // ReferenceError
  console.log(err.message); // nonvariable is not defined
  console.log(err.stack); // callstack with functions calls
  
  // Can also show an error as a whole
  // The error is converted to string as "name: message"
  console.log("Error obj", err); // ReferenceError: nonvariable is not defined
}
// ***************************************************************************
// Rethrowing
function parseUser(json) {
  try {
    return JSON.parse(json);
  } catch (err) {
    console.log("parseUser got error: ", err.message);
    throw err;
  }
}
parseUser("not a json");

// Use if/else to handle only errors that you know how to handle, and rethrow all others
function parseUser(json) {
  try {
    return JSON.parse(json);
  } catch (err) {
    if (err.name == "SyntaxError") {
      console.log("JSON Error: " + err.message);
    } else {
      console.log(err)
      // Rethrowing everything except SyntaxError
      throw err;
    }
  }
}
parseUser("not a json");


// ***************************************************************************
// Throwing our own errors
function parseUser(json) {
  let user = JSON.parse(json);
  if (!user.name) {
    throw new SyntaxError("Incomplete data: no name");
  }
  return user;
}
parseUser('{ "age": 30 }');

// It is strongly recommended to throw Error-compatible objects (with name, message, stack)
// For example: 
// throw {name: 'MyNiceError', message: 'Some nice description of what happened'}

// But actually you can throw anyghing, this is valid, but mostly useless
// throw true;

let myError = new Error()
myError.name = 'OwnError'
myError.message = 'Some nice message'
throw myError

try_catch_finally.js

// try/catch/finally

try {
  console.log("try");
  if (confirm("Make an error?")) {
    BAD_CODE();
  }
} catch (e) {
  console.log("catch");
} finally {
  console.log("finally");
}
// Finally block gets executed in ANY case - no matter error was or not

// finally and return
// What this code will print?
function finallyAndReturn() {
  try {
    return 1;
  } catch (error) {
    console.log(error);
  } finally {
    return 2;
  }
}
let res = finallyAndReturn();
console.log(res); // 2

/**
 Since finally executed AFTER try in ANY case, 
 return from finally will overwrite try/catch block return
*/

function catchAndReturn() {
  try {
    return 1;
  } catch (error) {
    return 2;
  } finally {
    return 3;
  }
}
let res = finallyAndReturn();
console.log(res); // 3

// Same here - finally replaces value of all other previous returns
// Use finally to correctly close database connections, write logs,

// Try can be used without catch

try {
  nondef;
} finally {
  console.log("i still work!");
}

// In this case error will be thrown outside, but finally will be executed