Why does the "+" operator consider the number as string when added

Question

Why does the + operator consider the number as string when added

Ex :

'3' + 4 + 5;  // "345"
 3 + 4 + '5'; // "75"

Answer 1

When using + with 2 numbers: Math.

When using + with a string: Concatenation.

3 + 4 = 7
7 + '5' = '75'

Answer 2

+ will only add two numbers if it has a number on the left hand side and a number on the right hand side.

'3' + 4 + 5;

First '3' + 4 has a string on the left hand side. So it converts the right hand side to a string and concatenates them.

Second '34' + 5 has a string on the left hand side. So it converts the right hand side to a string and concatenates them.

3 + 4 + '5';

First 3 + 4 has a number on both sides, so it adds them. Second 7 + '5' has a string on the right hand side, so it converts the left hand side to a string and concatenates them.

Answer 3

It's simple rule in javascript :

string + number = string (operation work as a string)
'3' + 4 + 5; = 345
7 + '5' = 75
number + number = number (operation work as a number)
3 + 4 = 7

Answer 4

Regarding the "why", you've already gotten answers, the way to fix it, in case you don't know or if it can help somebody else:

var x = parseInt('3') + 4 + 5;