I'm using JavaScript map for getting all values from this type of data:

var data = {
    size: 600,
    sectors: [
        {
            percentage: 0.7,
            label: 'Thing 1'
        },
        {
            percentage: 0.3,
            label: "Thing Two"
        }
    ]
}

and in data.sectors.map( function(item,key) I'm calculating angle using percentage. Right know my question is how to save value from map to add this value to next calculation with map?

What I've got is every time map run I recive angle to calculate function for example:

1: 25
2: 30
3: 25
4: 20

And this is the angle which I use to calculate one function but in other function I have to calucalte angle using also previus value of it:

1: 25
2: 55
3: 80
4: 100

How to get this using map in JS?

I'm using JavaScript map for getting all values from this type of data:

var data = {
    size: 600,
    sectors: [
        {
            percentage: 0.7,
            label: 'Thing 1'
        },
        {
            percentage: 0.3,
            label: "Thing Two"
        }
    ]
}

and in data.sectors.map( function(item,key) I'm calculating angle using percentage. Right know my question is how to save value from map to add this value to next calculation with map?

What I've got is every time map run I recive angle to calculate function for example:

1: 25
2: 30
3: 25
4: 20

And this is the angle which I use to calculate one function but in other function I have to calucalte angle using also previus value of it:

1: 25
2: 55
3: 80
4: 100

How to get this using map in JS?

• javascript
• html
• css
• variables

• You want to update data.sectors.map in data with map, right? – Adam Azad Commented Jan 20, 2017 at 7:24
• I'm updating data.sectors with map using push, but I have to get value from first in calculating map for example A and add this in a new calculation. Like in first move var A = 10 and in second we have got a new A = 25 but I need to add previues A to this A to get A = 35 to calculate in second 'loop' and in third 'loop' A = 35 + A – Nikalaj Commented Jan 20, 2017 at 7:28

2 Answers 2

I understand this question is older, but I just ran into a similar problem and somehow couldn't find other Q/A's regarding this.

By using all three arguments in Array.prototype.map()'s callback: currentValue, index, array you are able to update the current value by using the previous value(s), as follows:

let newValues = [25,30,25,20].map((curr, i, array) => {
  return array[i] += array[i-1] ? array[i-1] : 0
})

console.log(newValues)
// expected: [25,55,80,100]

Note, similar to Array.prototype.reduce() you need to set an initial value when array[i-1] does not exist (in the first iteration of map). Otherwise, you use the previous value.

Javascript map function is used where you want to map each value of the array to a new value, it returns a new array. If what you want to do is to get a single value from the array, for example, add all the values of the array, you should use Javascript reduce function.

data.sectors.reduce((accumulator, each)=>{
   return (accumulator + each )
})

https://developer.mozilla/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce

Use this for more insight into the reduce function.