When talking about JavaScript, it is important that you use the correct terminology. This way, if you talk about code with others, they'll understand what you mean without any ambiguity.
A "variable" is a place where you can store information, such as a string, or a number.
Variables are "declared" using the let and const keywords (or the older var keyword):
let x = 5;Here, we say: "declare variable x and initialize it with the number 5".
All variables have a type. In our example above, the variable x is a number. JavaScript supports the following types:
string, e.g. "HackYourFuture"number, e.g. 5, or 10.6boolean, e.g.trueorfalsearray*, e.g.[1, 2, 3]or['what', 'is', 'your', 'name']object, e.g.{name: 'John', age: 24}, or the special objectnullfunction, e.g.function () { return 4; }symbol
In addition, a variable may be undefined. This is also a special type.
To get the type of a variable, use the following code:
let x = 5;
let typeOfX = typeof x; // -> "number"Note that I've put an asterisk behind 'array'. That is because in JavaScript, array is a special kind of object:
let arr = [1, 2, 3];
let typeOfArr = typeof arr; // -> "object"However, in our communication, we will call these variables arrays.
The values null and undefined are very similar in JavaScript, but they behave a bit differently. The difference is that null always has type "object", and undefined always has type "undefined".
Whenever you declare a variable, but you don't set a value, the variable will become undefined. JavaScript will never make a variable null unless you explicitly program it.
let x;
console.log(typeof x); // -> "undefined"Variables that are arrays contain a list of things, instead of just one thing. What's inside the array, we typically call "elements". So, the array [1, 2, 3] has three elements. The array [] has no elements and is therefore empty. The number of elements in an array is called its "length".
When you want to access an element inside an array, you use an "index". This is the number that you put between brackets ([]).
Given the following code:
let arr = ['john', 'jane', 'jack'];
console.log(arr[0]);The number 0 is the "index of the first element of array arr". Conversely, the element "at index 0 in array arr is 'john'".
Instead of a number, you can also use a variable to access elements in an array, as long as this variable is a number:
let arr = ['john', 'jane', 'jack'];
let a = 1;
console.log(arr[a]); // -> janeIf the index you use is not an integer (a whole number), or if it's less than 0 or if it's greater than or equal to the array's length, you will get back undefined.
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!
let ageKey = 'age';
console.log(obj[ageKey]); // -> 24Remember 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 writearr['length']to access thelengthproperty 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.
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).
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. let 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"Note
Technically, this means that the string
swill become thethisspecial 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); // -> 9Here, 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();
...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 + 5This 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. let 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)aa > 4 ? "yes" : "no"a + ba && b || carr.lengthobj["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:
let-> this is a keyword, see belowlet x;-> this is a statement+-> this is only an operatorif (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.
Some words in JavaScript are special, e.g. var, if, while, return. These are called "keywords". You typically cannot use these words as names for your variables, functions.