Javascript and weakly defined types

Javascript is a weakly typed language, a variable can be equal to another if they have the same value even if they are different types.

As a result, Javascript has two equality operators;
== checks if the values of the compared objects are equal
=== checks if the values and types of the objects are equal

var fiveAsText = '5';
var fiveAsNumber = 5;

fiveAsText == fiveAsNumber // outputs true
fiveAsText === fiveAsNumber // outputs false

This can lead to some unusual behaviour when dealing with different types. For instance, when you perform mathematical operations on a string type with a numeric value

var five = '5';
var three = 3;

five + three // "53", string concatenation interprets the '5' as a string and overrides the addition operator
five - three // 2, subtraction interprets the '5' as a number

Update (28th August 2015)

A perfect example of the confusion that this can cause is as below
Javascript number/string casting

The explanation isn''t exactly intuitive but at least shows that there is some sort of logic behind it all

+ is overloaded as both the addition and string concatenation operator.

In mixed operations:

+ casts numbers to strings.

- casts strings to numbers.

Both are used to denote signage. Leading either on a string will cast it to a number, with that sign.

Pretty simple stuff, right?


Number('5') - 3
'5' + (3).toString()
'5' + (5).toString()
'foo' + Number('bar').toString()
'5' + (-Number('2')).toString()
// Long ass sign flipping, same as above
Number(('5' + (3).toString())) - 3
(Number('5') - 3) + 3

The above explanation is courtesy of /u/RewindHoodie from Reddit''s /r/ProgrammerHumor