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JavaScript

Posted by Jordan Stokes
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Jordan Stokes
Full Stack JavaScript Techdegree Graduate 27,292 Points

Why "+=" and not "+" for the score handler

I can't understand this line here:

score: prevState.players[index].score += delta

Why is it "+=" and not "+." I tried the same code with just "+" and it didn't work. However, I don't see any reason why it wouldn't or how "+=" could even work.

If I were to create an object and assign a value within to the sum of two separate values, this is how I would do it:

let value1 = 2; let value2 = 2;

let object = { thing: value1 + value2 }

console.log(object.thing);

And the output would be "4" without the "+="

1 Answer

Aayush Mitra
Aayush Mitra
24,904 Points
Aayush Mitra
Aayush Mitra
24,904 Points

When we use += we are adding a value to the existing value. For example:

let num1 = 40;
num1 += 1;
//num1 now equals 41

We use + when we are adding to values and putting it into a new variable. For example:

let num1 = 40;
let num2 = 60;
let num3 = num1 + num2;
//num3 is equal to 100

Hope this helps! If you need a more in depth explanation please let me know. Thanks! :)

Jordan Stokes
.a{fill-rule:evenodd;}techdegree seal-36
Jordan Stokes
Full Stack JavaScript Techdegree Graduate 27,292 Points

I do understand the nature of + vs +=. My confusion was based on a specific situation using Express. As far as I could see, I was using += in a situation where a variable wasn't yet created:

handleScore = (index, delta) => {
    this.setState( prevState => ({
      score: prevState.players[index].score += delta             // this line right here
    }));
  }

Video Link: https://teamtreehouse.com/library/update-state-based-on-a-players-index

However, I believe I figured it out. The point is to not just set the "score" in the new object, but to also set "score" in the "prevState.players" object.