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Typescript Sessions > Fundamentals > Flashcards

Fundamentals Flashcards

1
Q

What 3 keywords are used to declare variables?

A

let, var & const

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2
Q

What is the main difference between let/const and var?

A

let is block-scoped (only exists within the lifetime of a scope) & cannot be redeclared. Var is global scoped, and can be redeclared. Let is preferable to var.

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3
Q

What is the difference between let & const?

A

let declares a variable, const declares a constant. The value associated with a constant cannot be changed - it is immutable.

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4
Q

Why should const be the default keyword?

A

Using const offers a guarantee that is a variable should not be changed, it won’t be, which makes code more secure. Let then becomes used deliberately for variables that you plan to alter.

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5
Q

What are the basic/scalar types?

A 
number
string
boolean
undefined
object (rarely used)
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6
Q

What are the two simple types of data collections?

A

object

array (/ array-like)

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7
Q

What does the term ‘execute’ mean?

A

To execute means to activate the action defined by the code.

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8
Q

What is a function?

A

A function is a scope of code to be executed when it is invoked.

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9
Q

What format is number data-type stored as?

A

64-bit floating point numbers. There are no explicit scalar integer types in TS/JS.

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10
Q

How would you declare a constant variable “foo” with a value of 5?

A 
const foo = 5; // implicit
const foo: number = 5; // explicit
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11
Q

What is the difference between implicit and explicit typing?

A

The type of the variable if implicit is inferred from the value assigned upon initialisation, if explicit is the specified type.

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12
Q

What is a literal?

A

Any value that is not stored in a variable (e.g. 5, “hello”, true).

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13
Q

Define a variable “bar” that can be a string or a number, and initialise with value 5.

A

let foo: string | number = 5;

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14
Q

What does Truthy & Falsy mean?

A

Truthy values are values of any type that, in any operation that requires a boolean value, evaluate as True. Falsy values in the same situation evaluate to false.

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15
Q

What are the standard Falsy values?

A

false, 0, “”, undefined, null, NaN

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16
Q

If foo is a number variable of 5, what would foo.toFixed(2) produce?

A

toFixed returns a string version of a number to fixed decimal places.
foo.toFixed(2) === “5.00”;

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17
Q

If foo is a number variable of 555, what would foo.toPrecision(2) produce?

A

toPrecision defines the precision (rounding) of a number.

foo.toPrecision(2) === “560”;

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18
Q

what does the toString() method do?

A

Returns a string version of the variable.

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19
Q

If foo is a number variable of 5, what would foo.toString(2) produce?

A

toString returns a string version of the number in this case, with the number defining the base (default base 10). In this case it would be “101”;

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20
Q

What is a method?

A

A method is a function bound to an object/class.

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21
Q

What are the prototypes associated with each scalar type?

A

Number, String, Boolean, Object, Array

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22
Q

If foo is a number variable of 5, what would Number.isInteger(foo) produce?

A

Number.isInteger() is a static method on the Number wrapper class that evaluates if a number is an integer. Number.isInteger(foo) === true;

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23
Q

How are string literals represented?

A

With single or double quotes.
I recommend using double quotes for strings, and single quotes for chars, as this is standard for other programming languages.

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24
Q

What are escape characters?

A

characters in a string that are “escaped” with a backslash () that do something to that string representation in the browser. e.g. \n will create a newline.

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25
Q

What are template literals?

A

Template Literals are represented by backticks and evaluate values in ${} to string values within the literal.
e.g. Hello ${person.name}

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26
Q

What String methods can be used to return a string variable to lower and upper case?

A

foo. toUpperCase();

foo. toLowerCase();

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27
Q

What property defines the length of a string?

A

foo.length;

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28
Q

What method can be used to separate a string into an array?

A

foo.split(“ “) - this will split the words by spaces.

29
Q

What method can be used to return a proportion of a string

A

foo.slice(0, 5); – this would return “Hello” if foo is “Hello World”

30
Q

What method can be used to find the index of a character within a string

A

foo. indexOf(“a”);

foo. lastIndexOf(“a”); – searches from end first

31
Q

What method can be used to replace instances of a string with a different string?

A

foo. replace(“a”, “b”) // for 1

foo. replaceAll(“a”, “b”) // for all

32
Q

What are the arithmetic (number) operators

A 
\+ -> addition
- -> subtraction
* -> multiplication
/ -> division
% -> modulus or remainder of division
\++ -> increment (x = x + 1)
-- -> decrement (x = x - 1)
33
Q

What are the assignment operators?

A

Assignment operators define how a value on the right hand side, when evaluated, is assigned to the left hand variable.
= -> is assigned to
+= -> addition assignment (x = x + y)
-= -> subtraction assignment (x = x - y)
*= -> multiplication assignment (x = x * y)
/= -> division assignment (x = x / y)
%= -> modulus assignment (x = x % y)

34
Q

What are the 2 valid string operators?

A

+ -> String concatentation (“hello “ + “world”)

+= -> addition assignment (foo += “world”

35
Q

What are the comparison operators

A 
Operators that compare 2 values and output a boolean result.
===  -> is equal to
!==  -> is not equal to
>  -> is greater than
>=  -> is greater than or equal to
<  -> is less than or equal to
<= -> is less than or equal to

(== -> is loosely equal to… but don’t use)

36
Q

What are the logical operators?

A

&& -> Logical AND (evaluates to True if both LH & RH are truthy)
|| -> Logical OR (evaluates to true if one or both values are truthy)
! -> negates/inverts the boolean value of an expression (e.g. !true === false)

37
Q

How do logical AND & OR operators work

A

They execute the code on either side dependent on whether either side is truthy or falsy.

38
Q

How can logical assignment be implemented?

A

Logical or can be used to only assign
let foo = “” || “Hello World”; // outputs Hello World.
let foo ||= “Hello World”; // if foo is falsy

let bar = "I think" && "therefore I am"; // outputs "therefore I am"
let bar &&= "therefore I am" // if bar is truthy
39
Q

What is the nullish operator?

A 
The nullish operator (??) can be used to execute/evaluate the right hand side if the left hand is not nullish (null or undefined ONLY). There is also a nullish assignment operator.
let bar = "I think" ?? "therefore I am";
let foo = null ?? "I won't be executed";
40
Q

What does the typeof operator return?

A

the typeof operator will return the basic scalar type (object if not scalar) in string form of a value.
typeof “hello” === “string”;

41
Q

What does the instanceof operator return?

A

the instance of operator evaluates whether or not an object is an instance of a class.

42
Q

What are the main non-scalar Classes used in Typescript

A 
Array
Object
Date
JSON
Error
Global/window (node/browser)
Regexp (Regular Expressions)
43
Q

What is the syntax of a regular expression literal

A

/Hello World/igm -> will match a sequence of characters (in this case, exactly).

44
Q

What is a conditional

A

A conditional defines a scope of code to be executed if the input value evaluates to true or truthy.

45
Q

What is the syntax of an IF statement

A 
if (condition) {
  // if condition is true, this code will execute
}
46
Q

What is the syntax of an IF ELSE statement

A 
if (condition) {
  // if condition true
} else {
  // if condition false
}
47
Q

What will be the output of this conditional be? If (!!true) { console.log(“Option A”); } else { console.log(“Option B”) }

A

“Option A” -> !!true is double negation so evaluates to true, therfore the if codeblock will execute

48
Q

What is a loop?

A

A code block that repeatedly executes all the while an associated condition is true.

49
Q

What is the syntax of a WHILE loop?

A 
while (condition) {
   // while condition is met
}
50
Q

What is the syntax of a DO WHILE loop?

A 
do {
  // will run at least once and while condition is met
} while (condition)
51
Q

Will the following execute, if so how many times?

while (0) { console.log(“hey”) }

A

No, as 0 is falsy.

52
Q

Will the following execute, if so, how many times?

do { console.log(“hey”); } while (0)

A

Yes, once. Do executes the code once, and the condition associated with while is falsy.

53
Q

What type of loop is a FOR loop

A

A FOR loop is an iterative loop. (While is purely conditional)

54
Q

What is the syntax of a FOR loop

A 
for (start; end; onEach) {
  // executes on each iteration
}
55
Q

Write a for loop that logs out the first 5 characters in a string variable “foo”.

A 
for (let i = 0; i < 5; i++) {
   console.log(foo[i]);
}
56
Q

What does a FOR IN loop do?

A

Loops through the property keys in an object.

57
Q

What is the Typescript syntax of a FOR IN loop?

A 
for (let key in obj) {
  const typedKey = key as keyof typeof obj;
  console.log(obj[key]);
}
58
Q

What does a FOR OF loop do?

A

Loops through the property values in an object, or values of an array. NOTE: With an object, the method must be defined, as the values are not ordered. It is best not to use this with an object, there are ways around it.

59
Q

What is the syntax of a FOR OF loop?

A

let foo = [1,2,3];
for (let val of foo) {
console.log(val)
}

60
Q

What is the Date library?

A

The Date library gives a series of methods that converts the number of milliseconds since 1970 into dates and times.

61
Q

Which Date method gives the number of milliseconds since 1970.

A 
const foo = new Date();
foo.getTime(); // 1617545873697 or similar
62
Q

What are the two static methods available on the JSON class library?

A

JSON.parse() -> Transforms JSON to object

JSON.stringify() -> Transforms object to JSON.

63
Q

What does the keyof operator do?

A

The keyof operator takes an object type and produces a string or numeric literal union of its keys.

64
Q

What does the tsconfig file do?

A

Defines how Typescript is compiled.

65
Q

What does Typescript compile to?

A

Javascript.

66
Q

What does a bundler do?

A

A bundler simplifies file directories for runtime. Bundlers include Webpack, snowpack, parcel & rollup.

67
Q

How do you initialise a tsconfig file?

A

tsc –init (or touch tsconfig.json for blank slate). NOTE: use my personal tsconfig and save it as a snippet.

68
Q

How do you initialise a linter for Typescript

A

npx eslint –init

Then follow instructions and select typescript, and select style guide.

Typescript Sessions (23 decks)

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