How do I compare arrays in JavaScript? [duplicate]

Question

Can someone please describe the outcome of these expressions?

  [1, 2, 4] < [1, 2, 5]  // true
  [1, 3, 4] < [1, 2, 5]  // false


  [1, 2, 3] === [1, 2, 3]   // false
  [1, 2, 3] <   [1, 2, 3]   // false
  [1, 2, 3] ==  [1, 2, 3]   // false
  [1, 2, 3] >   [1, 2, 3]   // false


  [1, 2, 3] <= [1, 2, 3]   // true
  [1, 2, 3] >= [1, 2, 3]   // true

Thanks for help and fast answer !

Answer 1

Equality =

[1, 2, 3] == [1, 2, 3]

is described under The Abstract Equality Comparison Algorithm, which basically says

(if x or y are primitives, compare them, otherwise...)
Return true if x and y refer to the same object. Otherwise, return false.

Since different object literals always represent different objects, even if the content is the same, the above comparison fails.

Relational operators < >

Relative comparisons are different from equality. When you use < or >, arrays are compared as strings.

[1, 2, 4] < [1, 2, 5] 

The Abstract Relational Comparison Algorithm converts both operands to primitives. If an operand is an object, ToPrimitive calls [[DefaultValue]] which in turn is the same as obj.valueOf().toString(). Since valueOf of an object is the object itself, the whole thing boils down to

"1,2,4" < "1,2,5"

A common assumption that arrays are compared element-wise is not true:

[10,1,3] < [101,5]  // false

Note that valueOf can be overridden to affect the behavior of relational operators:

> a = [1,2,3]
[1, 2, 3]
> a < [1,2,4]
true
> a.valueOf = function() { return 'zzz' }
function () { return 'zzz' }
> a < [1,2,4]
false