javascript - Capture the current value of a variable - Stack Overflow

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I have a variable myVariable which is continuously changing. At some point I want to capture the value of myVariable (not a reference to myVariable) into another variable myVariableAtSomePoint.

Example code:

var myVariable = 1;

function test () {
    var myVariableAtSomePoint= myVariable;
    console.log(myVariableAtSomePoint);
}

myVariable = 2;

test(); // Prints 2, instead of 1.

I have a variable myVariable which is continuously changing. At some point I want to capture the value of myVariable (not a reference to myVariable) into another variable myVariableAtSomePoint.

Example code:

var myVariable = 1;

function test () {
    var myVariableAtSomePoint= myVariable;
    console.log(myVariableAtSomePoint);
}

myVariable = 2;

test(); // Prints 2, instead of 1.

• javascript

Share Improve this question Follow edited Oct 19, 2011 at 14:55 Randomblue asked Oct 19, 2011 at 14:32 RandomblueRandomblue 116k150150 gold badges362362 silver badges557557 bronze badges 11

• Yes, it stores a number (integer). – Randomblue Commented Oct 19, 2011 at 14:34
• 1 What is it about myVariableAtSomePoint = myVariable; that won't work? – Pointy Commented Oct 19, 2011 at 14:35
• 1 If you are using jQuery: var myVariableAtSomePoint = jQuery.extend({}, myVariable);. If not, copy the source code/idea for jQuery.extend from jQuery :P – Esailija Commented Oct 19, 2011 at 14:37
• 1 Well you said it's just "a number (integer)". – Pointy Commented Oct 19, 2011 at 14:51
• 2 The problem you are having is not about references. You are calling test after you changed value. The code inside the test function is not executed until you call the function. – Felix Kling Commented Oct 19, 2011 at 15:15

 |  Show 6 more ments

5 Answers 5

Sorted by: Reset to default 4

You mention in your ments that myVariable is a number. Since myVariable houses a primitive type, simply use the code below:

myVariableAtSomePoint = myVariable;

Take a look at JAVASCRIPT: PASSING BY VALUE OR BY REFERENCE. Here's a quote:

When passing in a primitive type variable like a string or a number, the value is passed in by value.

I would also suggest reading: How do I correctly clone a JavaScript object?

EDIT:

I believe that you're assuming that the placement of the function in the code affects the value of the variables. It does not. See examples below:

This:

function test () {
    var myVariableAtSomePoint= myVariable;
    console.log(myVariableAtSomePoint);
}

myVariable = 2;

test(); // Prints 2, instead of 1.

Is the same as this:

var myVariable = 1;

function test () {
    var myVariableAtSomePoint= myVariable;
    console.log(myVariableAtSomePoint);
}

myVariable = 2;

test(); // Prints 2, instead of 1.

Your problem is that you're changing the value of myVariable before you're assigning it to myVariableAtSomePoint. For this to work as you want, you'll need to call the test() function before you change the value of myVariable

var myVariable = 1;

function test () {
    var myVariableAtSomePoint= myVariable;
    console.log(myVariableAtSomePoint);
}

test(); // Prints 1
myVariable = 2;
test(); // Prints 2

IMPORTANT: No matter the placement of the function, the code inside test() is not executed until you call the function.

The variable only holds the reference and nothing else. If you want a copy of the object that the variable is pointing to, you will need some way to copy that object. Either you can implement a copy method on the object itself (if you know what properties it has) or you can iterate through all properties on the object using a for ... in loop and copy them to a newly allocated object. Something like:

//o is the object you want to copy
var o2 = {}; //gives a new object
for (x in o) {
   o2[x] = o[x];
}

Your code:

var myVariable = 1;

function test () {
    var myVariableAtSomePoint= myVariable;
    console.log(myVariableAtSomePoint);
}

myVariable = 2;

test(); // Prints 2, instead of 1.

You assign 2 to myVariable, and then assign myVariable (which has now value of 2) to myVariableAtSomePoint via test() so of course it is 2. You don't need any magic copying here (since numbers are primitive) just assigment is enough.

You'll just need to copy each property on the object, of course if there are sub-objects they might get passed by reference as well:

function copy(o) { //could be called "clone"
  var i, n;
  n = {};
  for (i in o)
  {
    if (o.hasOwnProperty(i)) {
      n[i] = o[i];
    }
  }
  return n;
}

You can do something this to do exactly what you wanted. It is slightly more involved however.

var myVariable = 1;

var test = null;
(function() {
  const myVariableAtSomePoint = myVariable;
  test = function() {
    console.log(myVariableAtSomePoint);
  }
})();

myVariable = 2;

test(); // Prints 1

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