JavaScript Expressions

Expressions are units of code that can be evaluated and resolve to a value. Expressions in JS can be divided in categories.

• Arithmetic expressions
• String expressions
• Primary expressions
• Array and object initializers expressions
• Logical expressions
• Left-hand-side expressions
• Property access expressions
• Object creation expressions
• Function definition expressions
• Invocation expressions

Arithmetic expressions

Under this category go all expressions that evaluate to a number:

1 / 2
i++
i -= 2
i * 2

String expressions

Expressions that evaluate to a string:

'A ' + 'string'

Primary expressions

Under this category go variable references, literals and constants:

2
0.02
'something'
true
false
this //the current object
undefined
i //where i is a variable or a constant

but also some language keywords:

function
class
function* //the generator function
yield //the generator pauser/resumer
yield* //delegate to another generator or iterator
async function* //async function expression
await //async function pause/resume/wait for completion
/pattern/i //regex
() // grouping

Array and object initializers expressions

[] //array literal
{} //object literal
[1,2,3]
{a: 1, b: 2}
{a: {b: 1}}

Logical expressions

Logical expressions make use of logical operators and resolve to a boolean value:

a && b
a || b
!a

Left-hand-side expressions

new //create an instance of a constructor
super //calls the parent constructor
...obj //expression using the spread operator

See the spread operator tutorial

Property access expressions

object.property //reference a property (or method) of an object
object[property]
object['property']

Object creation expressions

new object()
new a(1)
new MyRectangle('name', 2, {a: 4})

Function definition expressions

function() {}
function(a, b) { return a * b }
(a, b) => a * b
a => a * 2
() => { return 2 }

Invocation expressions

The syntax for calling a function or method

a.x(2)
window.resize()