JavaScript: Why does push not work on these objects in loop?

Question

I am trying to store all the powerful numbers until 100 (numbers that can be expressed as powers, or more formally, https://en.wikipedia.org/wiki/Powerful_number) into an array, then log (Store) it.

I have created two arrays, one with the description of the power used to arrive there and one just numbers.

The code starts by assuming that no base can be larger than 32, as the square root of 1000 is ~31.6. Then, another loop is formed, to determine the exponent. As 2 to the 10th is larger than 1000, the largest exponent must be less than 10.

The algorithm works, and I was able to remove duplicates.

I tried to print numArr, and it worked flawlessly. All 40 of the powerful numbers less than or equal to 1000 were stored. However, when I try to print the objArr, which is the result array, the array is simply 50 objects of the last iteration of the loop that was less than 1000, 31 squared.

Why is this?

var objArr = [];
var numArr = [];
var tempNum = 0;
var tempObj = {
  number: 0,
  power: ""
};

function shouldAppend(number) {
  if (number > 1000) {
    return false
  }
  return true;
}


// the base of the power
for (var b = 2; b <= 32; b++) {
  // the exponent of the power
  for (var y = 2; y <= 10; y++) {
    // tempNum is the result of the base raised to the exponent
    tempNum = Math.pow(b, y);
    // checks if "tempNum" is less than 1000
    if (shouldAppend(tempNum)) {
      // all of the below only exceutes if the tempNum is less or equal to 1000
      // sets the temporary object's number property to the temporary number
      tempObj.number = tempNum;
      // sets the temporary object's power property to the current iterators
      tempObj.power = `${b} to the power of ${y}`;
      // logs the temporary object
      console.log(tempObj);
      // pushes the temporary object to the result array
      objArr.push(tempObj);
      numArr.push(tempNum);
      // logs the current result array
      console.log(objArr);
    }
  }
}

You are changing the same object over and over. pushing an object onto an array does not create a new object — Patrick Evans
– Patrick Evans, Commented Sep 8, 2017 at 4:16
There is only a single tempObj in your code. objArr.push(tempObj) will not copy the object, but just store the reference to that single object. Create a new object from a literal inside your loop whenever you need it. — Bergi
– Bergi, Commented Sep 8, 2017 at 4:16
@JeroenHeier Yes! So this is because I am storing the object reference, not copy inside the array? — Eamon Ma
– Eamon Ma, Commented Sep 8, 2017 at 4:19

Hugo Silva · Accepted Answer · 2017-09-08 04:16:37Z

1

You are pushing the same object on every iteration.

Try cloning the object inside the loop, before pushing:

objArr.push(Object.assign({}, tempObj));

answered Sep 8, 2017 at 4:16

Hugo Silva

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Comments

Michael Snower · Accepted Answer · 2017-09-08 04:21:32Z

You need to create a new object each loop iteration. You simply need to instantiate your tempObj within the loop. This code should work.

        var objArr = [];
        var numArr = [];
        var tempNum = 0;

        function shouldAppend(number) {
          if (number > 1000) {
            return false
          }
          return true;
        }
        // the base of the power
        for (var b = 2; b <= 32; b++) {
          // the exponent of the power
          for (var y = 2; y <= 10; y++) {
            var tempObj = {
              number: 0,
              power: ""
            };
            // tempNum is the result of the base raised to the exponent
            tempNum = Math.pow(b, y);
            // checks if "tempNum" is less than 1000
            if (shouldAppend(tempNum)) {
              // all of the below only exceutes if the tempNum is less or equal to 1000
              // sets the temporary object's number property to the temporary number
              tempObj.number = tempNum;
              // sets the temporary object's power property to the current iterators
              tempObj.power = `${b} to the power of ${y}`;
              // logs the temporary object
              console.log(tempObj);
              // pushes the temporary object to the result array
              objArr.push(tempObj);
              numArr.push(tempNum);
              // logs the current result array
              console.log(objArr);
            }
          }
        }

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