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0 = false

1 = true

| `&&` |0|1|

|------|-|-|

|0|0|0|

|1|0|1|

| `\|\|` |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

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.

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"]`.

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;

}

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.

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"

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();

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.

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.

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.