YXD - 4 months ago 27

Node.js Question

Say I want to sum

`a.x`

`arr`

`arr = [{x:1},{x:2},{x:4}]`

arr.reduce(function(a,b){return a.x + b.x})

>> NaN

I have cause to believe that a.x is undefined at some point.

The following works fine

`arr = [1,2,4]`

arr.reduce(function(a,b){return a + b})

>> 7

What am I doing wrong in the first example?

Answer

After the first iteration your're returning a number and then trying to get property `x`

of it to add to the next object which is `undefined`

and maths involving `undefined`

results in `NaN`

.

try returning an object contain an `x`

property with the sum of the x properties of the parameters:

```
var arr = [{x:1},{x:2},{x:4}];
arr.reduce(function (a, b) {
return {x: a.x + b.x}; // returns object with property x
})
```

Explanation added from comments:

The return value of each iteration of `[].reduce`

used as the `a`

variable in the next iteration.

Iteration 1: `a = {x:1}`

, `b = {x:2}`

, `{x: 3}`

assigned to `a`

in Iteration 2

Iteration 2: `a = {x:3}`

, `b = {x:4}`

.

The problem with your example is that you're returning a number literal.

```
function (a, b) {
return a.x + b.x; // returns number literal
}
```

Iteration 1: `a = {x:1}`

, `b = {x:2}`

, `// returns 3`

as `a`

in next iteration

Iteration 2: `a = 3`

, `b = {x:2}`

returns `NaN`

A number literal `3`

does not (typically) have a property called `x`

so it's `undefined`

and `undefined + b.x`

returns `NaN`

and `NaN + <anything>`

is always `NaN`