How to create an array of objects from a 2d array in Javascript?

Question

This is analogous to my actual problem but i feel illustrates the problem i need to solve without complicating things.

I need to create an array of objects by iterating through a 2d array. The 2d array looks something like the"sampleArray" below.

let sampleArray = [ 
  ["open",2], 
  ["high",3], 
  ["low",5], 
  ["close", 10], 
  ["volume", 20], 
  ["open",20], 
  ["high",30], 
  ["low",50], 
  ["close", 100], 
  ["volume", 21],
  ["open",21], 
  ["high",33], 
  ["low",51], 
  ["close", 1], 
  ["volume", 2],
  ["open",7], 
  ["high",8], 
  ["low",5], 
  ["close", 11], 
  ["volume", 22]
 ];

I'm currently formatting a particularly ugly set of API data and have flattened the data down to a 2d array. What i need now is to put these values into an array of 5 objects where each object's data property is populated by all of the values with that particular label.

newObject = [
    {
     data: [2,20,21, ...]
     label: "open"  
    }, 
    {
     data: [3,50, ...]
     label: "high"
    }, 
    {
     data: [etc...]
     label: "low"  
    }, 
    {
     data: [etc...]
     label: "close"  
     }, 
    {
     data: [etc...]
     label: "volume"  
     }
]

I'm about 3 hours into trying to make this work and feel that I'm going about this all wrong, I keep getting an error that newArray[i]["label"] is undefined, I get why it's happening (no object with that property exists YET, I'm just not sure how to express that idea (If the object with that label doesn't exist yet, create it and add that matching value to it's value property) in javascript.

function makeArray(array) {
  let newArray = [];
  for(let i = 0; i <= array.length; i++){
    if(newArray[i]["label"] !== array[i][0]){
      newArray.push({
        "label": array[i][0],
        "value": array[i][1]
      })
    }    
    else{
      newArray.push({
        "value": array[i][1]
      })
    }
  }
return newArray;
}

let test = makeArray(sampleArray);
console.log(test);

Sorry for the long post, I've been flattening this nightmare API all day and feel like I'm just running in place at this point. I would much prefer a point in the right direction than an outright answer. I'm gonna hit the sack and see if someone can provide me with a little perspective by morning. Thank you!

Answer 1

You can use reduce

let sampleArray = [
  ["open", 2],
  ["high", 3],
  ["low", 5],
  ["close", 10],
  ["volume", 20],
  ["open", 20],
  ["high", 30],
  ["low", 50],
  ["close", 100],
  ["volume", 21],
  ["open", 21],
  ["high", 33],
  ["low", 51],
  ["close", 1],
  ["volume", 2],
  ["open", 7],
  ["high", 8],
  ["low", 5],
  ["close", 11],
  ["volume", 22]
];


let newObject = Object.values(sampleArray.reduce((c, [n, v]) => {
  c[n] = c[n] || {label: n,data: []};
  c[n].data.push(v);
  return c;
}, {}));

console.log(newObject);

Answer 2

Using Vanilla JS:

let sampleArray = [
  ["open", 2],
  ["high", 3],
  ["low", 5],
  ["close", 10],
  ["volume", 20],
  ["open", 20],
  ["high", 30],
  ["low", 50],
  ["close", 100],
  ["volume", 21],
  ["open", 21],
  ["high", 33],
  ["low", 51],
  ["close", 1],
  ["volume", 2],
  ["open", 7],
  ["high", 8],
  ["low", 5],
  ["close", 11],
  ["volume", 22]
];

function transform(sampArr) {
  let obj = {};                                 // A map equivalent to store repeated value
  
  // Loop through each array within array
  sampArr.forEach(function(arr) {
    if (!obj[arr[0]]) {
      obj[arr[0]] = {};                         // Instantiate the map the first time
    }

    if (!obj[arr[0]].data) {
      obj[arr[0]]["data"] = [];                 // First time instantiate the array
    }

    obj[arr[0]].data.push(arr[1]);              // Push the repeated values
  });


  // Create the structure you want
  return Object.keys(obj).map(function(key) {
    return {
      data: obj[key].data,
      label: key
    };
  });
}

console.log(transform(sampleArray));