REVIEW JavaScript week 2

This review covers:
• Recap Logical operators
• Typeof
• Loops (for/while)
• Functions 
• Advanced data types [Objects]  
• Conditions
• Statements vs Expressions 
• Naming conventions

Recap Logical operators:

0 = false
1 = true

AND &&

&&	0	1
0	0	0
1	0	1

OR ||

||	0	1
0	0	1
1	1	1

So you can say that false in combination with && always returns true

true && false //-> false
false && true //-> false
false || true //-> true
true || false //-> true

Typeof

typeof always returns the data type in a string.

So for example:

let bar = 42; 
typeof bar    //-> 'number' 
typeof typeof bar; //->  'string'

So the data type of what typeof returns is always a string, bar on the other hand is still a number.

Objects

Variables that are objects also contain a list of things, but instead of them being in some specific order, they can be assigned to words, called "keys". Instead of "elements" the things that are inside objects are called "properties".

let obj = {name: 'John', age: 24};

This object has two properties: name and age. The "value" of the property name is the string 'John'. The "value" of the property age is the number 24.

When accessing object properties, you can use the dot-notation: obj.name or the bracket-notation: obj["name"]. Note that the latter looks a lot like the way to access array elements. However, here what's inside the bracket (called "key" for objects, instead of "index") must be a string.

console.log(obj.name); // -> 'John'
console.log(obj['name']); // -> 'John'

Just like with arrays, you can also use a variable to access properties, as long as these variables are strings. In this case you cannot use the dot-notation!

var ageKey = 'age';
console.log(obj[ageKey]); // -> 24

Remember that there is a very big difference between obj[name] and obj["name"].

Note:

Thinking back of arrays, the length of an array can be retrieved by arr.length. So as mentioned before, arrays are just like other JavaScript objects. You could even write arr['length'] to access the length property of the array. JavaScript will look: is what we put between brackets a number? Then it is an index and we'll look up the correct array element. If it's a string, it's a key and we will look up the corresponding property.

Functions

A function is a reusable piece of code. Functions are very important in JavaScript, to the extent that some people call JavaScript a "function-oriented" language. As mentioned above, variables can be of type function. In fact, every function is a variable.

The following two pieces of code have the exact same result:

function sum(a, b) {
  return a + b;
}

and

let sum = function (a, b) {
  return a + b;
}

Note

This is not entirely true, as in the second code, the function is "anonymous", i.e. it has no name. But in both cases, you can call the function like this: sum(4, 5).

Parameters & arguments

When writing function sum(a, b), a and b are the "parameters" of the function. We say that this function has two parameters. (Sometimes, you'll see the word "arity": this function has "arity" 2, but that is something you don't have to use for now.)

Now, when calling function sum, e.g. var s = sum(4, 5);, we say that the numbers 4 and 5 are the "arguments" of the function. Arguments are "passed" to the function: "we pass 4 and 5 to the function sum".

So remember the difference between the word "parameter" and "argument". Many people confuse them, and that's not a big problem, but understanding the difference is always nice:

• A parameter is the name you want to give to the variable that is available inside of the function.
• An argument is the actual value you want to assign to the parameters when you call the function.

A function that "has two parameters" is also said to "take/accept two arguments". But, sometimes you'll hear people say: "the function has two arguments" or "the function takes two parameters". While formally incorrect, you'll know what they mean.

Calling a function on something

In JavaScript, you can call functions on something. By this, we mean that you use the dot to call the function. For instance, when we say "call method trim on string s", we mean:

let s = " this is a string  ";
s.trim(); // -> "this is a string"

Note

Technically, this means that the string s will become the this special variable inside of the function.

However, there are functions that you don't call on anything:

function sum(a, b) { return a + b; }
sum(4, 5); // -> 9

Here, you call the function sum on nothing.

Most built-in functions in JavaScript, like math functions or logging functions, also use the dot:

Math.round(4.5);
console.log("hello");
Array.from([1, 2, 3]);

Indeed, these functions are also called "on" Math, console, Array, and so on. However, in this case, their purpose is more to group them logically, so here it's not very important to use that terminology. We'd rather say: "call the function Math.round with 4.5 as an argument", i.e. we include it in the full name of the methods.

It's more when you think about which functions you can call "on" your own variables (strings, arrays, numbers, etc):

myString.trim();
myArray.slice();
myNumber.toString();
...

Statements & expressions

Most programming languages that you'll encounter in real life are called "imperative" programming languages. JavaScript is such an imperative programming language. Imperative is another word for command-like. That is, you give the computer a bunch of commands after each other. First do this, then do that, etc.

These individual commands are called "statements" in imperative programming languages. You can compare them with sentences in the English language. They have a use by themselves, and do not need something else. "The man eats bread." is a full sentence, it conveys a meaning by itself. A sentence in English is always terminated by a period.

Similarly, a statement in JavaScript should provide a command by itself. JavaScript-statements are (almost always) terminated by a semicolon.

This is a complete statement:

let s = "HackYourFuture";

It is a full command: declare a variable s and initialize it with "HackYourFuture". JavaScript doesn't need any other information to know what we want. The statement is terminated with a semicolon.

However, this is not a complete statement:

4 + 5

This equals 9, but what is JavaScript to do with it? It doesn't provide a command. You'd need to do something with it, e.g. var x = 4 + 5; or callFunction(4 + 5). We call these parts of statements "expressions". Expressions are not terminated by semicolons. Expressions always "evaluate into a value". In our example, the expression 4 + 5 "evaluates into 9". If expressions cannot be evaluated into a value, they are invalid. For instance, 4 + is not a valid expression, it is incomplete, because we need something else after the plus sign.

So, statements can contain expressions. Can expressions contain statements? No, they cannot. However, they can themselves contain expressions. Think about 4 + 5: it contains the expressions 4 and 5, as these both evaluate into a value: the expression 4 evaluates into the number 4, it is a very simple expression. Similarly, true, null, undefined are all expressions.

Examples of expressions

Here are some examples of expressions. Remember: expressions evaluate into a value, but do not provide a command:

• sum(a, b)
• a
• a > 4 ? "yes" : "no"
• a + b
• a && b || c
• arr.length
• obj["name"]
• [1, 2, 3]
• arr[1]
• [1] (this is an array with one element!)
• function a() { return 4; }

The last one requires a bit of explanation. If you write:

function a() { return 4; }

by itself, this is a statement (a function declaration statement). However, if you write it as part of a statement, such as:

let b = function a() { return 4; }

now it is an expression. This is an exceptional situation where something can be a statement or an expression.

Examples of not-expressions

The following are not expressions:

• var -> this is a keyword, see below
• var x; -> this is a statement
• + -> this is only an operator
• if (a > 4) { return "yes"; } else { return "no"; }

if is also a statement. However, it is quite a complex statement. It is also referred to as a "construct", just like for, while, try, etc.