Why does having a + before a string in JavaScript convert it to an Integer?

Question

Why does the following snippet of code convert my variable of type string to type number?

let stringInteger = '42';
let convertToInteger = +stringInteger;
console.log(typeof convertToInteger)

More specifically, why does prefixing + to the variable have this effect? Note, I'm asking why not what it does.

Possible duplicate of What does = +_ mean in JavaScript

Romeo Sierra
– Romeo Sierra

2018-05-07 02:12:51 +00:00
Commented May 7, 2018 at 2:12 — Romeo Sierra
– Romeo Sierra, Commented May 7, 2018 at 2:12

Daniel · Accepted Answer · 2018-05-07 02:15:34Z

2

It's the Unary Plus Operator.

Your question is answered here:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators#Unary_plus_()

answered May 7, 2018 at 2:15

Daniel

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Keri M. · Accepted Answer · 2018-05-07 03:10:26Z

1

It's called a Unary Plus operator. It basically tries to convert non-integer variables into integers (ie +'true' and +'false' can be 1 and 0). You can read more about it on MDN and you can read more about the differences between this and other ways to parse integers in js here.

edited May 7, 2018 at 3:10

answered May 7, 2018 at 2:23

Keri M.

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