Statements

One instruction at a time

A program is a list of statements. The engine runs them top to bottom, one after another, and each one does a single piece of work.

let total = 0;
total = total + 5;
total = total * 2;

console.log(total);

10

Order matters. Move the multiplication above the addition and the answer changes, because each statement works on whatever the ones before it left behind.

Semicolons

A semicolon ends a statement. JavaScript will insert missing ones for you, a rule called automatic semicolon insertion, and it mostly guesses right. Mostly is the problem:

function bad() {
  return
  {
    ok: true
  };
}

console.log(bad());

undefined

A semicolon was inserted straight after return, so the object below it is unreachable and the function returns nothing. Put the brace on the same line as return and it works.

Blocks

Curly braces group statements into a block, and anywhere one statement is allowed a block can go instead.

const hour = 9;

if (hour < 12) {
  console.log('Morning');
  console.log('Two statements, one block');
}

Morning
Two statements, one block

Braces are optional for a single statement after if, and leaving them out has caused enough real bugs that most teams require them anyway.

Whitespace

JavaScript ignores extra spaces, tabs and blank lines between statements, so indentation is for humans rather than the engine. Two spaces per level is the common choice and the one used here.

Case matters

Names are case sensitive. total, Total and TOTAL are three different names, and mixing them up produces a quiet bug rather than an error.

const price = 10;

console.log(price);
console.log(typeof Price);

10
undefined