I want to create a new object.

An object that adds only values ​​of the same key.

I want to make it in a way that adds num1 to each other and num2 to num2.

The created object has a value of num1 : 4.

But I don't know why the value is NaN

this is my code.

let mydata = [
    {num1: 1 , num2: 2 , num3: 3, num4: 4},
    {num1: 1 , num2: 2 , num3: 3, num4: 4},
    {num1: 1 , num2: 2 , num3: 3, num4: 4},
    {num1: 1 , num2: 2 , num3: 3, num4: 4},
    ]
 
    const sum = {};

  for(let prop of mydata){
    for(let key in prop){
      sum[key]  = prop[key];
    }
  }
  
 console.log(sum);
 //{ num1: NaN, num2: NaN, num3: NaN, num4: NaN } => why Nan?? 

CodePudding user response：

sum[key] default value is undefined, undefined a number = NaN, so you need to assign 0 for sum[key] initially

let mydata = [{
    num1: 1,
    num2: 2,
    num3: 3,
    num4: 4
  },
  {
    num1: 1,
    num2: 2,
    num3: 3,
    num4: 4
  },
  {
    num1: 1,
    num2: 2,
    num3: 3,
    num4: 4
  },
  {
    num1: 1,
    num2: 2,
    num3: 3,
    num4: 4
  },
]

const sum = {};

for (let prop of mydata) {
  for (let key in prop) {
    if (!sum[key]) {
      sum[key] = 0;
    }
    sum[key]  = prop[key];
  }
}

console.log(sum);

.as-console-wrapper {
  max-height: 100% !important;
  top: 0;
}

CodePudding user response：

sum[key] is initially undefined, and undefined number yields NaN.

You can fix this by assigning a value of zero to sum[key] if its value is falsy:

for(let prop of mydata) {
  for(let key in prop) {
    sum[key] = sum[key] || 0;
    sum[key]  = prop[key];
  }
}

Or if you want to condense things further:

for(let prop of mydata) {
  for(let key in prop) {
    sum[key] = (sum[key] || 0)   prop[key];
  }
}