Javascript, saving values from onclick and saving previous values in arrays

Question

i'm trying to save values retrieved from "onclick" and also save the values previously retrieved from "onclick" in different arrays when buttons are clicked.

but it seems like when "for loop" is used, newly retrieved values overwrite previously retrieved data even though those values are saved separately in different arrays.

i'm really confused right now, does anybody know why?

(if you hit the button "refresh", you can see the current values that are saved.)

var firstValue = [];
var preValue = [];



function returneD(a){

    preValue = firstValue;
    console.log("returned! preValue:  "+preValue);

    for (var i = 0; i < 1; i++) {
      firstValue[i] = a;
      console.log("returned! firstValue:  "+firstValue);
    }
}


function refresh1(){
  console.log("preValue:  "+preValue);
  console.log("firstValue:  "+firstValue);
}

<!DOCTYPE html>
<html>
<head>
  <script src="jstest.js"></script>
</head>
<body id="thebody">

<button id="1" onclick="returneD(this.id)">O N E</button>
<button id="2" onclick="returneD(this.id)">T W O</button>
<button id="3" onclick="returneD(this.id)">T H R E E</button>
<button id="4" onclick="returneD(this.id)">F O U R</button>
<br>
<button onclick="refresh1()">refresh</button>

</body>
</html>

preValue = firstValue; doesn't create a copy; it only causes preValue to point at the same array in memory. Try this instead: preValue = firstValue.splice(); — user5734311
– user5734311, Commented Mar 28, 2017 at 12:17

Prabu · Accepted Answer · 2017-03-28 12:32:52Z

Please refer this code will work,

firstValue is array which already stored value after that this will assign old value to preValue array.

var firstValue = [];
var preValue = [];



function returneD(a){

    preValue = a;
    console.log("returned! preValue:  "+preValue);

    for (var i = 0; i < 1; i++) {
      firstValue[i] = a;
      console.log("returned! firstValue:  "+firstValue);
    }
}


function refresh1(){
  console.log("preValue:  "+preValue);
  console.log("firstValue:  "+firstValue);
}

<!DOCTYPE html>
<html>
<head>
  <script src="jstest.js"></script>
</head>
<body id="thebody">

<button id="1" onclick="returneD(this.id)">O N E</button>
<button id="2" onclick="returneD(this.id)">T W O</button>
<button id="3" onclick="returneD(this.id)">T H R E E</button>
<button id="4" onclick="returneD(this.id)">F O U R</button>
<br>
<button onclick="refresh1()">refresh</button>

</body>
</html>

Erdem E. · Accepted Answer · 2017-03-28 12:30:46Z

1

In JavaScript Arrays are just like Objects. When you assign: preValue = firstValue; this simply takes a reference to the variable hence you fail to copy the value.

In order to achieve that, you need to copy each value of that array to the previous values array. So, you can run another for loop for first to previous assignment:

for (var i = 0; i < 1; i++) {
  prevValue[i] = firstValue[i];
}

Assuming they are of the same size.

answered Mar 28, 2017 at 12:30

Erdem E.

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