Why does adding an array to number return a string? [duplicate]

Question

var array = [1,2,4];

array+1  //gives '1,2,41'.

Can anyone explain this behaviour?

Answer 1

Can anyone explain this behaviour?

This answer attempts to explain this behavior from the point of view of spec.

As per spec, during the run-time evaluation of +, both expressions (left and right) are converted to their primitive values.

7. Let lprim be ToPrimitive(lval).
8. Let rprim be ToPrimitive(rval).

toPrimitive tries to pass hint:number (since invoked during arithmetic evaluation) to OrdinaryToPrimitive

3. If hint is "string", then
   a. Let methodNames be «"toString", "valueOf"».
4. Else,
   b. Let methodNames be «"valueOf", "toString"». //this gets invoked

Since one of the values were casted to string via 4a) above, string concatenation takes place.

Hence

[1,2,4] + 1 => [1,2,4].toString() + "1" => "1,2,4" + "1" => (finally) "1,2,41"

Answer 2

Array is casted to string - then concatenated with integer value which is also casted to string.

Answer 3

When you use the + sign with a declared javascipt object (var array), even if one of the elements is a number, it doesn't perform an arithmetic addition operation - it concatenates the values as two strings.

In your example, your array [1,2,4] is being casted into a string with a value of 1,2,4. So 1,2,4 concatenated with 1 is 1,2,41