Javascript: expressions and variables

I was recently studying Javascript out of curiousity as this language is used so much in the current web and mobile development. These are notes on basic Javascript knowledge to better memorize its syntax.

Main sources for learning that I use are fantastic book Eloquent Javascript and this interesting course on Domestika.

Expressions

Javascript is a C-family language, so basic arithmetic operations are exactly the same as in C:

3 + 8;  // Sum, 11
2 * 5;  // Product, 10
7 % 3;  // Remainder, 1

as well as logical operations:

5 > 2; // true
7 < 5; // false
9 <= 10;  // true
20 >= 20; // true

Equality can be checked with == operator but this operator thinks that zero is equal to false (that is, it does type casting), while special operator === does not do this, it does not change the types. It is recommended to use the latter operator most of the time.

Variables and constants

Variables can be defined with the let keyword. Pre-2015 Javascript used the word var, which can also be used still but let is recommended.

let a = 25;

In the above code, the variable a is declared and initialized to value 25.

One can also declare a variable without initializing it:

let b;

then it has special type undefined.

Constants (that is, variables that cannot change their values) are defined using keyword const:

const MEANING_OF_LIFE = 42;

It is customary to use upper case letters and underscores for constants.

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