Javascript +运算符vs-运算符

Question

I am puzzled by this snippet :

var n1 = 5-"4";
var n2 = 5+"4";

alert(n1);
alert(n2);

​I understand that n1 is 1. That is because a minus operator would convert the string "4" into number and subtract it from 5. But why do we get 54 in case of + operator?

Can someone explain this difference between + and = operators to me?

Answer 1

By type conversion any + expression, that contains a strings, will result in a string. Thus all operands (in your case 5 ) will be converted to a string, before executing the concatenation.

- on the other hand is just an arithmetic operand, thus "4" is converted to an integer and the calculation is performed as you expect.

Answer 2

It's because in n2 , + is being treated as concatenation, not addition. So 5 is converted to the string "5" and "4" is concatenated, giving "54".

Answer 3

When there's a string in either side of + , the + will be considered as a string concatenating operator, the other side will be converted to string and then do the concatenating.

And be careful of something like 1+2+'3' , the result is '33' rather than '123' .

Answer 4

- operator has only one meaning - numbers subtraction (or negation and in that case, also conversion to number). In case of + operator, however, there are two: number addition and strings concatenation. When one of the operands of + operator is a string it does string concatenation instead of numbers addition.

The entire process is a bit more complicated than that though and involves an algorithm that you can learn a bit more here , for example.

Answer 5

The + operator is also a string operator. Quite every basic type variable in javascript can be interpreted also in its string representation. You are just attaching 5 to 4 getting 54.

The - operator is not a string operator so the compiler tries to interpret "4" as a number, thus getting 1

Answer 6

Javascript takes 5 as a number and "4" as string. The javascript + operator use to concat two things. If you want to addition please use parseInt.

var n1 = 5-"4";
var n2 = parseInt(5)+parseInt("4");

alert(n1);
alert(n2);