multidimensional array - What does +a + +b mean in JavaScript? - Stack Overflow

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There was an answer to a nested array problem on stack overflow that involved a return statement +a + +b. (no period) What is this? What does it do? It seems to add, but I'm not sure how it does that. How does it differ from a + b? The code is below, and it works:

var array= [1, 2, [3, 4], [], [5]];
var sum = array.toString().split(",").reduce(function(a, b) { return +a + +b; 
});

console.log(sum);

There was an answer to a nested array problem on stack overflow that involved a return statement +a + +b. (no period) What is this? What does it do? It seems to add, but I'm not sure how it does that. How does it differ from a + b? The code is below, and it works:

var array= [1, 2, [3, 4], [], [5]];
var sum = array.toString().split(",").reduce(function(a, b) { return +a + +b; 
});

console.log(sum);

• javascript
• multidimensional-array

Share Follow asked Sep 1, 2018 at 22:32 ACFACF 9311 gold badge22 silver badges1010 bronze badges 3

• 6 The + in front coerces a variable to a number. – CertainPerformance Commented Sep 1, 2018 at 22:32
• 2 "5" + 6 === "56" whereas +"5" + 6 === 11 – ibrahim mahrir Commented Sep 1, 2018 at 22:38
• The + in front of a variable means the developer doesn't know what is going on but tries to calculate anyway, if possible. – Frederik.L Commented Sep 1, 2018 at 22:43

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5 Answers 5

Sorted by: Reset to default 10

The + before the variable is the unary + operator. You are probably familiar with the unary -, e.g. -x.

Both +x and -x convert x to a number, but -x also changes its sign. This is not the primary purpose of the existance of the unary +, but it is a nice sideeffect.

Therefore, the difference between a + b and +a + +b would be visible from this example:

x = '9';          // '9'
x = +'9';         // 9
x = '9' + '9';    // '99'
x = +'9' + +'9';  // 18

JavaScript is dynamically typed, but has no built-in operators for casting a value to ensure it is the right type for an operation. Consequently, various tricks are used when it's important that a value is treated as a number, or a string, etc.

In this case, the operator being used is the "unary +" which is the natural counterpart of the "unary -" that would be used to write a negative number: just as "-x" means "0 - x", "+x" means "0 + x". On a number, this has no effect, but other values will be converted to a number.

This is important because the "binary +" operator does different things depending on the type of its operands: if a was a string, a + b would mean "append two strings together" rather than "sum two numbers".

Given an imaginary cast_to_number function, the equation could be more readably written as return cast_to_number(a) + cast_to_number(b).

Assuming a or b were of type String, adding a + operator in front of it will coerce it to a number.

This ensures that ’1’ + ‘1’ will be 2 (addition of numbers) and not 11 (concatenation of strings).

javascript has a unary operator + and -, both of them converts the operand to a number type, but the - operator negate the result.

for example:

> -'20'
-20
> +'500' + +'600'
1100

The + unary operator is used for explicit coercion in JavaScript.

Even though some developers look down upon it, saying var num = 3 + +c; is rather ugly especially to people not aware of the coercion at hand, potentially even "error-like" looking piece of code.

The most mon practical use of the + unary operator is probably to get a number timestamp of the date

var timestamp = +new Date(); // this works!

Perhaps a good piece of memorablia is the - unary operator, which for some reason our human brain seems much happier to interpret. Basically, if we see var x = -c; we presume the piler:

1. Attempts to do a value to number coercion of the variable c
2. If it can return a number, flip the sign and return the negative number (-)

The + unary operator does the same thing, only without the sign flipping.

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