Expressions are single units the engine can evaluate. Statements are bigger steps: they can contain expressions, and the engine runs them to do work.

Programs are built from statements. A statement can span several lines.

JavaScript groups statements into a few families:

expression statements
declaration statements
control flow statements
loop statements
miscellaneous statements

Separating statements

Statements can end with an optional semicolon ;. Semicolons let you put multiple statements on one line. I usually skip semicolons, but either style works.

Expression statements

An expression on its own is also a statement:

2
0.02
'something'
true
false
this //the current scope
undefined
i //where i is a variable or a constant
1 / 2
i++
i -= 2
i * 2
'A ' + 'string'
[] //array literal
{} //object literal
[1,2,3]
{a: 1, b: 2}
{a: {b: 1}}
a && b
a || b
!a
object.property //reference a property (or method) of an object
object[property]
object['property']
new object()
new a(1)
new MyRectangle('name', 2, {a: 4})
function() {}
function(a, b) { return a * b }
(a, b) => a * b
a => a * 2
() => { return 2 }
a.x(2)
window.resize()

Declaration statements

A declaration assigns a value to a variable name.

Examples:

var i = 0
let j = 1
const k = 2

//declare an object value
const car = {
  color: 'blue'
}

Here are function declarations:

//declare a function
function fetchFromNetwork() {
  //...
}
//or
const fetchFromNetwork = () => {
  //...
}

Control flow statements

Statements can be grouped in a block:

{
  //this is a block
  const a = 1;
  const b = 2;
}

Use a block when JavaScript expects one statement but you need several.

Conditional statements evaluate an expression first. Depending on the result, they run one statement or block:

if (condition === true) {
  //execute this block
} else {
  //execute this block
}

You can omit curly braces when you only have one statement:

if (condition === true) /* statement */ else /* another statement */

We cover every control flow structure in the next lessons.

Loop statements

Loops work like if statements. Some loops repeat while an expression stays true. Others walk a list and run a statement for each item.

See my full JavaScript loops tutorial.

Miscellaneous statements

return

Returns a value from a function and stops execution there.

throw

Throws an exception. We cover exceptions in a later lesson.

try and catch

A try/catch block catches exceptions.

try {

} catch (<expression>) {

}

use strict

This statement applies strict mode.

debugger

Adds a breakpoint the debugger can use.

JavaScript Statements