Syntax

Values

JavaScript deals in two sorts of value: fixed ones you write down, called literals, and ones held by a name.

console.log(42);
console.log('a string literal');
console.log(true);
console.log([1, 2, 3]);
console.log({ key: 'value' });

42
a string literal
true
[1, 2, 3]
{key: "value"}

Names

A name is attached to a value with const or let. From then on the name stands in for the value everywhere.

const country = 'Germany';
let visits = 3;

visits = visits + 1;

console.log(country, visits);

Germany 4

Operators

Operators combine values. Arithmetic ones produce numbers, comparison ones produce true or false, and assignment stores a result.

console.log(7 + 3);
console.log(7 > 3);
console.log('a' + 'b');

10
true
ab

Expressions and statements

An expression is anything that produces a value. A statement is an instruction. 2 + 2 is an expression; const four = 2 + 2; is a statement containing one.

The difference matters because expressions can go wherever a value is expected, including inside other expressions.

const width = 4;
const height = 3;

console.log('area: ' + (width * height));

area: 12

Keywords

Some words are reserved by the language and cannot be used as names: const, let, function, return, if, class and around thirty more. Trying to use one is a syntax error, which at least fails loudly.

Identifiers

A name may contain letters, digits, _ and $, and may not start with a digit. Convention is camelCase for variables and functions, PascalCase for classes, and capitals for values that are fixed for the life of the program.

const firstName = 'Ada';
const MAX_USERS = 100;

console.log(firstName, MAX_USERS);

Ada 100