# JavaScript specials

Domains:

This chapter briefly recaps the features of JavaScript that we've learned by now, paying special attention to subtle moments.

## Code structure

Statements are delimited with a semicolon:

alert('Hello'); alert('World');


Usually, a line-break is also treated as a delimiter, so that would also work:

alert('Hello')


That's called "automatic semicolon insertion". Sometimes it doesn't work, for instance:

alert("There will be an error after this message")


Most codestyle guides agree that we should put a semicolon after each statement.

Semicolons are not required after code blocks {...} and syntax constructs with them like loops:

function f() {
// no semicolon needed after function declaration
}

for(;;) {
// no semicolon needed after the loop
}


...But even if we can put an "extra" semicolon somewhere, that's not an error. It will be ignored.

More in: info:structure.

## Strict mode

To fully enable all features of modern JavaScript, we should start scripts with "use strict".

'use strict';


The directive must be at the top of a script or at the beginning of a function.

Without "use strict", everything still works, but some features behave in the old-fashion, "compatible" way. We'd generally prefer the modern behavior.

Some modern features of the language (like classes that we'll study in the future) enable strict mode implicitly.

More in: info:strict-mode.

## Variables

Can be declared using:

• let
• const (constant, can't be changed)
• var (old-style, will see later)

A variable name can include:

• Letters and digits, but the first character may not be a digit.
• Characters \$ and _ are normal, on par with letters.
• Non-Latin alphabets and hieroglyphs are also allowed, but commonly not used.

Variables are dynamically typed. They can store any value:

let x = 5;
x = "John";


There are 7 data types:

• number for both floating-point and integer numbers,
• string for strings,
• boolean for logical values: true/false,
• null -- a type with a single value null, meaning "empty" or "does not exist",
• undefined -- a type with a single value undefined, meaning "not assigned",
• object and symbol -- for complex data structures and unique identifiers, we haven't learnt them yet.

The typeof operator returns the type for a value, with two exceptions:

typeof null == "object" // error in the language
typeof function(){} == "function" // functions are treated specially


More in: info:variables and info:types.

## Interaction

We're using a browser as a working environment, so basic UI functions will be:

prompt(question[, default]) : Ask a question, and return either what the visitor entered or null if they pressed "cancel".

confirm(question) : Ask a question and suggest to choose between Ok and Cancel. The choice is returned as true/false.

alert(message) : Output a message.

All these functions are modal, they pause the code execution and prevent the visitor from interacting with the page until they answer.

For instance:

let userName = prompt("Your name?", "Alice");
let isTeaWanted = confirm("Do you want some tea?");

alert( "Tea wanted: " + isTeaWanted ); // true


## Operators

JavaScript supports the following operators:

Arithmetical : Regular: * + - /, also % for the remainder and ** for power of a number.

The binary plus + concatenates strings. And if any of the operands is a string, the other one is converted to string too:

alert( '1' + 2 ); // '12', string
alert( 1 + '2' ); // '12', string


Assignments : There is a simple assignment: a = b and combined ones like a *= 2.

Bitwise : Bitwise operators work with integers on bit-level: see the docs when they are needed.

Ternary : The only operator with three parameters: cond ? resultA : resultB. If cond is truthy, returns resultA, otherwise resultB.

Logical operators : Logical AND && and OR || perform short-circuit evaluation and then return the value where it stopped.

Comparisons : Equality check == for values of different types converts them to a number (except null and undefined that equal each other and nothing else), so these are equal:

alert( 0 == false ); // true
alert( 0 == '' ); // true


Other comparisons convert to a number as well.

The strict equality operator === doesn't do the conversion: different types always mean different values for it, so:

Values null and undefined are special: they equal == each other and don't equal anything else.

Greater/less comparisons compare strings character-by-character, other types are converted to a number.

Logical operators : There are few others, like a comma operator.

More in: info:operators, info:comparison, info:logical-operators.

## Loops

• We covered 3 types of loops:

		// 1
while (condition) {
...
}

// 2
do {
...
} while (condition);

// 3
for(let i = 0; i < 10; i++) {
...
}
• The variable declared in for(let...) loop is visible only inside the loop. But we can also omit let and reuse an existing variable.

• Directives break/continue allow to exit the whole loop/current iteration. Use labels to break nested loops.

Details in: info:while-for.

Later we'll study more types of loops to deal with objects.

## The "switch" construct

The "switch" construct can replace multiple if checks. It uses === (strict equality) for comparisons.

For instance:

let age = prompt('Your age?', 18);

switch (age) {
case 18:
alert("Won't work"); // the result of prompt is a string, not a number

case "18":
break;

default:
alert("Any value not equal to one above");
}


Details in: info:switch.

## Functions

We covered three ways to create a function in JavaScript:

1. Function Declaration: the function in the main code flow

		function sum(a, b) {
let result = a + b;
return result;
}
2. Function Expression: the function in the context of an expression

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

Function expressions can have a name, like sum = function name(a, b), but that name is only visible inside that function.

3. 		// expression at the right side
let sum = (a, b) => a + b;

// or multi-line syntax with { ... }, need return here:
let sum = (a, b) => {
// ...
return a + b;
}

// without arguments
let sayHi = () => alert("Hello");

// with a single argument
let double = n => n * 2;
• Functions may have local variables: those declared inside its body. Such variables are only visible inside the function.
• Parameters can have default values: function sum(a = 1, b = 2) {...}.
• Functions always return something. If there's no return statement, then the result is undefined.
Function Declaration Function Expression
visible in the whole code block created when the execution reaches it
- can have a name, visible only inside the function

More: see info:function-basics, info:function-expressions-arrows.

## More to come

That was a brief list of JavaScript features. As of now we've studied only basics. Further in the tutorial you'll find more specials and advanced features of JavaScript.

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