What is the purpose of a plus symbol before a variable?
What does the +d in
function addMonths(d, n, keepTime) {
if (+d) {
mean?
Solution 1:
The + operator returns the numeric representation of the object. So in your particular case, it would appear to be predicating the if on whether or not d is a non-zero number.
Reference here. And, as pointed out in comments, here.
Solution 2:
Operator + is a unary operator which converts the value to a number. Below is a table with corresponding results of using this operator for different values.
| Value | + (Value) |
|---|---|
| 1 | 1 |
| '-1' | -1 |
| '3.14' | 3.14 |
| '3' | 3 |
| '0xAA' | 170 |
| true | 1 |
| false | 0 |
| null | 0 |
| 'Infinity' | Infinity |
| 'infinity' | NaN |
| '10a' | NaN |
| undefined | Nan |
| ['Apple'] | Nan |
| function(val){ return val } | NaN |
Operator + returns a value for objects which have implemented method valueOf.
let something = {
valueOf: function () {
return 25;
}
};
console.log(+something);
Solution 3:
It is a unary "+" operator which yields a numeric expression. It would be the same as d*1, I believe.
Solution 4:
As explained in other answers it converts the variable to a number. Specially useful when d can be either a number or a string that evaluates to a number.
Example (using the addMonths function in the question):
addMonths(34,1,true);
addMonths("34",1,true);
then the +d will evaluate to a number in all cases. Thus avoiding the need to check for the type and take different code paths depending on whether d is a number, a function or a string that can be converted to a number.