JavaScript Overview

Question 1

What is a program?

Answer 1

A set of instructions for a computer to carry out.

We write programs to solve a specific problem or provide a specific experience. The skill of programming, therefore, has two steps: first, programming is finding solutions to problems; second, programming is implementing those solutions in a particular programming language so your computer can understand and execute your solution.

Question 2

What is JavaScript?

Answer 2

JavaScript (JS) is a programming language available in all modern browsers. JS is an implementation of the ECMAScript standard.

We can use the JS programming language, which ECMA determines, to control the engine in a browser. The better you understand the programming language, the better you control the engine.

Question 3

What is ECMA?

Answer 3

ECMA controls the standard for JavaScript programming language, which is called ECMAScript. All web browsers should have a JavaScript version that is ECMA compliant. Each web browser is responsible for interpreting the ECMAScript.

Question 4

What is DOM API?

Answer 4

Document Object Model API

The part of JavaScript that deals with interacting with HTML elements. It’s specified by the W3C (World Wide Web Consortium), which is the same organization that creates the specification for HTML.

Question 5

What is Chrome DevTools?

Answer 5

Chrome DevTools provides an interactive JavaScript console where you can run and debug code.

Question 6

How do you open Chrome DevTools on a MAC?

Answer 6

Option + ⌘ + J

Question 7

How do you open Chrome DevTools on a PC?

Answer 7

Ctrl + Shift + J

Question 8

What is a variable?

Answer 8

A variable is a name that is attached to a value. Variables are used to create and represent data in our applications. (Variables are data in our apps.)

Question 9

How does a variable work?

Answer 9

When you need to create data, you create a keyword. Use the var, let, or const keywords to create the data. Then you enter how you want the JS to represent the piece data it’s grabbing:

var someIdentifier

Then you give it an assignment operator:

var someIdentifier = ‘somevalue’;

So here we are telling it to grab this piece of memory (var) with this label (someIdentifier) and set it equal to this value (somevalue).

You can also write it in this way with an undeclared variable:

var someIdentifier;
someIdentifier = somevalue;

Question 10

What is a constant?

Answer 10

A constant cannot be reassigned with a new value. It’s used to define constant values, which is another way of saying variables whose value cannot be reassigned.

You can declare it in JS by entering “const”:

const someIdentifier = ‘somevaluethatwon’teverchange’

The defining feature of a constant is that its value cannot be changed, and the browser enforces this behavior. When you try to change the value of a constant, you’ll get an error.

Question 11

In JavaScript, what are the 3 commands we can use to declare space in memory where we’ll store values?

Answer 11

var
let
const

Question 12

In JavaScript, what does the equal sign (=) mean?

Answer 12

Assignment operator

Question 13

What is let or var?

Answer 13

These are used to define variables with values that can (but don’t have to) be changed or reassigned.

Question 14

How can you declare a variable without an assignment operator? What does this do?

Answer 14

var someIdentifier;

When you declare a variable without an assignment operator, it has no value yet.

Question 15

What value does JavaScript give declared but unassigned variables?

Answer 15

undefined

Question 16

What are two rules for naming variables?

Answer 16

Reserved words: You can’t use reserved words in a variable name. For example, you can’t reassign the name var to a new rule like this:

var var = ‘some value’;

However, you can use a reserved word as part of a name though it might be redundant:

var varFoo;

Character restrictions: A variable name must begin with an upper or lower case letter, $, or _. It cannot begin with a number. Spaces cannot appear anywhere in the variable name. Comparison and logical operators cannot be used in variable names. So these are unacceptable:

let my Foo = ‘bar’;
let my+Foo;

Numbers can appear in a variable name, so this is acceptable:

let myFoo2;

Question 17

What are two important stylist guidelines when writing your code in JavaScript?

Answer 17

Use camelCasing: A camel-cased variable name starts with a lowercase letter, and then uses lowercase letters throughout, except for the first letter of any new words in the variable name after the first — for these first letters, use uppercase.

Use meaningful variable names: Choose names that reflect how the variable gets used in the program. A well-chosen variable name can help other people reading the code to understand how the variable is intended to be used.

Question 18

How can you force JavaScript to avoid global variables?

Answer 18

At the top of your JavaScript, enter this:

‘use strict’;

Question 19

What is a global variable, and why should you avoid it?

Answer 19

A global variable is when you enter the variable without the keyword like var or let. Here is an example:

foo = ‘bar’;

The proper way is this:

let foo=’bar’;

Global variables tend to make code buggier and harder to reason about.

Question 20

What is a data type?

Answer 20

A data type is a value that variables can have. Data types help us organize our data because there is a lot.

Question 21

How many data types are there and what are they?

Answer 21

There are 6 different data types:

String
Number
Boolean
Null
Undefined
Object

Note: Functions are not always considered a data type by everyone.

Question 22

What is coercion?

Answer 22

How variables are converted from one type to another in JavaScript. For example:

const stringVar = 'Kilroy was here ';
const numVar = 12;

const combined = stringVar + numVar;
typeof combined; // => string
console.log(combined); // => Kilroy was here 12

The example above demonstrates that if you combine a number with a string using the + operator, the resulting value will be a string.

Question 23

What is typeof in JavaScript?

Answer 23

It’s a built-in command JavaScript provides to see the data type of a particular value. So if you run typeof 2, the result would be “number”.

Question 24

What is the string data type?

Answer 24

Strings are used to represent text. They include opening and closing quotes (either single or double, but both must be the same kind — you can’t open with a single and close with a double). So. . .

let foo = ‘bar’;

and

let bar = “foo”;

are both valid.

Question 25

What is the number data type?

Answer 25

The number data type is used to represent numbers. This includes integer values (whole numbers like 1, 2, 3, 10000000…) and floating point decimal numbers (numbers like 1.234999).

Question 26

What is the null data type?

Answer 26

null is a special value used to indicate that a variable has no value.

Note: Be aware that if you run typeof null in JavaScript, you’ll get an eyebrow-raising answer: “object”. Even though null is a distinct data type, typeof says it’s an object. This is a relic of a much earlier version of JavaScript that was not fixed in ES5 or ES6.

Question 27

What is the Boolean data type?

Answer 27

Booleans signify truth and falsity. A boolean has two possible values: true and false.

let loggedIn = true;

if (!loggedIn) {
redirectToLoginScreen();
}

In the example above, we set loggedIn to true, and we then have a block of code that runs if and only if the user is not logged in. The ! in !loggedIn inverts the Boolean value of the item to the right of it, so !false evaluates to true and !true evaluates to false.

Question 28

What is the undefined data type?

Answer 28

undefined is a special value that the browser assigns to variables when it first creates them in memory. So this:

var foo;

would have a value as undefined in the JavaScript. You should never directly set a value to undefined. It’s fine to check if a variable is undefined, but you shouldn’t do let foo = undefined;. If you want to represent the absence of a value, use null.

Question 29

What is the object data type?

Answer 29

Objects allow us to have variables that don’t point to a single value (say, for instance, a single string) but instead to a collection of values. Here’s an example of a JavaScript object:

const person = {
  name: 'Jane Doe',
  greet: function() {
    console.log('Hello world');
  }
};

console. log(person.name); // => Jane Doe
person. greet(); // => Hello world

Here we’ve created an object called person that has a name property and a greet method.

Question 30

What is scope in JavaScript?

Answer 30

Scope is about how variables can be accessible (used, reused, and manipulated) within the code. There are two kinds of scope: global and block scope.

Question 31

What is variable scope?

Answer 31

Variable scope is a set of rules that define which parts of your code can access a particular variable. It’s what allows us to reuse variable names at different points in our code, without things breaking.

Question 32

What is global scope?

Answer 32

When you declare a variable in a global scope, that variable can be accessed anywhere in the code.
When a variable is declared outside of a function, it is in global scope.

Question 33

What is block scope?

Answer 33

When you declare a variable in a block scope, the variable is only accessible within the function. When the function is done with its instructions, the variable goes away.

Question 34

What is scope chain?

Answer 34

JavaScript understands scope through scope chain. Through scope chain, JavaScript looks at the name of the variable locally to see if it’s defined there.
If it’s not, then it looks at the parent scope of the function to see if it’s defined there. If it’s not, then it continues to search up the chain until it reaches the global variable to find the name.

Thus, if you declare a variable in a function which also has the same name as a global variable, JavaScript will use the local variable vs the global variable.

Question 35

Is it possible for global scope to extend across different files?

Answer 35

Yes, it is sometimes possible. If you declare a global variable in one file, you can refer to it in another. In order for this to work, the first file with the global variable needs to be loaded before the second file that is declaring the global variable from the first file.

Question 36

What is a side effect and when can it occur?

Answer 36

A side effect occurs when the a function goes outside of the local scope and into the parent scope and alters the code there. It can happen when you use global variables.

Question 37

What is a determinate and indeterminate code?

Answer 37

A determinate function is one that will always return the same value if it’s provided with the same inputs. An indeterminate function, in contrast, is one that — given a single set of inputs — returns one value some times and another at other times. This can lead to frustrating and hard-to-track-down bugs in your code. An indeterminate code can occur when you have side effects from using global variables.

Question 38

What is pure code?

Answer 38

Has determinate functions and no side effects.

Question 39

How do you prevent global scope?

Answer 39

Always use let or const when declaring your variables. Never declare a variable without these keywords.

Enable strict mode in the JavaScript. To do this, you enter ‘use scrict’; at the top of the JavaScript file.

Question 40

When is it okay to use global variables?

Answer 40

When you are in a JavaScript library like JQuery.