Adv Conc- Modules, Exceptions, Garbage Collection, Side effects/Pure Functions

Question 1

What are the benefits of using modules? (3 things)

Answer 1

If a program is contained entirely in one file, even just 100 lines of code can take time to understand fully. Splitting the program up is easier to understand each component.

Working on a program as a team is easier with multiple files.

You have to put in more work to encapsulate and make private data properly.

Question 2

Import a module managed by npm

Answer 2

const readline = require(‘readline-sync’);
let choice = readline.question(“Do you want to run this program? (y/n)”);

Question 3

CommonJS modules are compatile with browsers
t or f

Answer 3

Browsers don’t natively support CommonJS modules, but you can use a transpiler like Babel to transpile code that uses CommonJS modules into a format that can be used by browsers.

Question 4

Export and import a module

Answer 4

module.exports = anObject

let pear = require(“./CodeTester1”);
// it needs the path

Question 5

hat happens if you have multiple exports?

let apple = {
a: 1,
}
let orange = {
a: 2,
}
module.exports = apple;
module.exports = orange;

let pear = require(“./CodeTester1”);
console.log(pear.a);

Answer 5

The last module.export to get processed is processed

Question 6

export a module (for use in a compatible browser) (2 ways)

Answer 6

export let variable = primitive or object

or

export {variable}

Question 7

import a module (for use in a compatible browser)

Answer 7

import { variableNameFromOtherFile } from “./filePathAndName.ext”

Question 8

import multiple modules in one line (for use in a compatible browser)

Answer 8

import { variableName1, variableName2 } from “./filePathAndName.ext”

Question 9

Import a module and change the variable name (for use in a compatible browser)

Answer 9

import { variableName1 as newVar1, variableName2 } from “./filePathAndName.ext”

Question 10

Import all modules (for use in a compatible browser) and access them

Answer 10

File1.js
export function foo() {
console.log(1);
}

File2.js
import * as FooModule from “./File1”;
FooModule.foo(); // 1

Question 11

What is this called?

File1.js
export function foo() {
console.log(1);
}

File2.js
import * as FooModule from “./File1”;
FooModule.foo(); // 1

Answer 11

Namespace import.

While using Namespace imports is possible, it is more common and preferred to use Named imports for importing specific declarations.

Question 12

How many default exports are allowed per file

Answer 12

1

Question 13

Do default export and import (for use in a compatible browser

Answer 13

bar.js
function bar() {
console.log(2);
}
export default bar;

otherfile.js
import bar from “./bar”;
bar(); // 2

Question 14

Do default export and import and changed the name (for use in a compatible browser

Answer 14

bar.js
function bar() {
console.log(2);
}
export default bar;

otherfile.js
import renamedBar from “./bar”;
renamedBar(); // 2

Question 15

Do default export and import and changed the name
Also import a non default module
(for use in a compatible browser)

Answer 15

utils.js
export function foo() {
console.log(1);
}
function bar() {
console.log(2);
}
export default bar;

otherfile.js
import renamedBar, { foo } from “./utils.js”;
foo(); // 1
renamedBar(); // 2

Question 16

import and export are usually only compatible in browser.

Do it in node and make it compatible (2 ways)

Answer 16

change the file names to .mjs

Alternatively, a more preferred method is to add “type”: “module” to the package.json file. This enables ES6 modules and the files can retain the .js extension.

//package.json
{
“name”: “index”,
“version”: “1.0.0”,
“description”: “”,
“main”: “index.js”,
“type”: “module”,
“scripts”: {
“test”: “echo "Error: no test specified" && exit 1”
},
“keywords”: [],
“author”: “”,
“license”: “ISC”
}

Question 17

Create a custom error

Answer 17

throw new Error(“Divide by zero!”);

Question 18

Catch and error and do a block

Answer 18

try {
} catch(errorObject) {
}

Question 19

With a try catch statement, have it only do something if the error is of a certain kind

Answer 19

function divideOneBy(divisor) {
try {
let result = div(1, divisor);
console.log(result);
} catch (error) {
if (error instanceof DivideByZeroError) {
console.log(${error.message} ignored);
}
}
}

divideOneBy(1); // 1
divideOneBy(0); // Divide by zero! ignored

Question 20

With a try catch statement, have it only do something if the error is of a certain kind

If it’s another kind, let that error throw

Answer 20

function divideOneBy(divisor) {
try {
let result = div(1, divisor);
console.log(result);
} catch (error) {
if (error instanceof DivideByZeroError) {
console.log(${error.message} ignored);
} else {
throw error;
}
}
}

divideOneBy(1); // 1
divideOneBy(0); // Divide by zero! ignored

Question 21

What Happens When A Program Throws An Exception (3 things)

Answer 21

1. js goes back to look for a try block to see if it contains the code. Looks back at all the blocks, if none are try blocks, throws the error
2. if it finds a try block, it exits the try block (without executing anymore code) and executes the catch block
3. If the catch block re-throws the exception, it again resumes as before, looking for another try block.

Question 22

What happens with this:
try {
console.log(1);
a = b;
console.log(2);
} catch(error) {
console.log(3);
}

Answer 22

1
3

Question 23

YOu should use exceptions for flow control
“if this throws an error, then do this other code”
t or f

Answer 23

F

Question 24

What’s the idea of “ask forgiveness” in error throwing

Answer 24

– Computer programming pioneer Admiral Grace Hopper

Exceptions are sometimes likened to the “ask forgiveness” approach to error handling. Rather than asking whether we can do something, we just go ahead and try it. If an exception does occur, then we have to ask forgiveness – handle the exception or just accept the cost of an error.

In contrast to the “ask forgiveness” approach, the “get permission” approach involves determining beforehand whether something is going to work. For instance, if we expect a user to input a number, we can test whether they really did enter a number before we try to use it as a number. This is often harder to do than allowing an exception to be raised, but once permission is granted, we know things will work.

Question 25

In general, an exception handler in a catch block should do as little as possible. But you can… (5 things)

Answer 25

Ignore the exception.

Return an error value (e.g., undefined, null) to the calling function.

Set a flag that the program can test after the handler finishes running.

Log a simple error message

Throw another exception with an explicit throw statement. (but you can explicitly set another exception. Just don’t do anything like try to read a file that might now be there)

Question 26

What makes a function have side effects? (5 things)

Answer 26

It reassigns any non-local variable.

It mutates the value of any object referenced by a non-local variable.

It reads from or writes to any data entity (files, network connections, etc.) that is non-local to your program.

It raises an exception.

It calls another function that has any side effects that are not confined to the current function. For instance, if you call a function that mutates an argument, but that argument is local to the calling function, then it isn’t a side effect. (See the example with sort later in this assignment.)

Question 27

we will often talk about side effects as a general characteristic of some functions. If the function can have side effects when used as intended, then we say the function itself has side effects. In practice, functions that have side effects have them regardless of what arguments are passed in.

What makes a function not used as intended? (3 things)

Answer 27

If a required argument is omitted, the function isn’t being used as intended.

If you pass an array to a function that expects a number, the function isn’t being used as intended.

If you call a function before you’ve done any required preparations (such as opening a connection to a remote server), the function isn’t being used as intended.

Question 28

Does this have side effects?

function sortCopy(arrayOfNumbers) {
arrayOfNumbers = arrayOfNumbers.slice(); // creates a copy of an array
arrayOfNumbers.sort((a, b) => a - b);
return arrayOfNumbers;
}

Answer 28

no

// sort has local side effects

Question 29

do either of these have side effects?

[1, 2, 3].map(number => 2 * number);
[1, 2, 3].map(number => {
console.log(number);
return 2 * number;
});

Answer 29

[1, 2, 3].map(number => 2 * number);
[1, 2, 3].map(number => {
console.log(number);
return 2 * number;
});

Question 30

Does this have side effects?

function divideBy(numerator, denominator) {
try {
if (denominator === 0) {
throw new Error(“Divide by zero!”); // side effect: raises an exception
}
} catch (error) {
return undefined;
}

return numerator / denominator;
}

Answer 30

No, the error was handled

Question 31

Does this have side effects?
function divideBy(numerator, denominator) {
try {
if (denominator === 0) {
throw new Error(“Divide by zero!”); // side effect: raises an exception
}
} catch (error) {
console.log(error.message);
return undefined;
}

return numerator / denominator;
}

Answer 31

yes, the catch block has a side effect

Question 32

Does Math.random have side effects?

Answer 32

Math.random() has a side effect (it changes the state of the random number generator).

Question 33

What makes a pure function? (2 things)

Answer 33

Have no side effects.

Given the same set of arguments, the function always returns the same value during the function’s lifetime. This rule implies that the return value of a pure function depends solely on its arguments.

The consistent return value is possibly the most important feature of pure functions. The fact that the return value is dependent solely on the arguments implies that nothing else in the program can influence the function during the function’s lifetime.

Question 34

What s a functions lifetime?

Answer 34

A function’s lifetime begins when the function is created. It ends when the function is destroyed.

Question 35

Whats the benefit of pure functions?

Answer 35

A big benefit of pure functions is that the consistent return value and lack of side effects make them easy to test. Since they are effectively isolated from the rest of the program, you don’t have to worry about what happens elsewhere. Nothing outside of the function can have any effect on it. Nothing in the function can have any impact on the rest of the program. This is very convenient and helpful when testing.

Question 36

Update one object’s properties with the properties of another.
Exampel:
let obj1 = {
a: 1,
b: 2,
c: 3,
}

let obj2 = {
b: 1312,
}

we want b of obj1 to be 1312

Answer 36

Object.assign(obj1, obj2)

Question 37

Remove an object property

Answer 37

delete obj1.property

Question 38

Syntax for:
exporting/ importing in browser for regular variable,
at declaration,
or default,
default at declaration
multiple imports/exports and change name
multiple imports/exports and default and change name

Answer 38

export {apple}
import {apple} from ‘’

export let apple
import {apple} import {apple}

export default apple
import apple (or change name)

export default apple =
import apple (or change name)

export {apple, orange}
import {apple, orange}

export {pear, apple}
import {apple as orange, pear as fruit} from ‘./jstest2.js’;

let apple = 123;
let purple = ‘a’;

export default pear;
export {apple};
import purple, {apple} from “./jstest2.js”; (the default has to go first)