Does JavaScript pass by reference?

Primitives are passed by value, and Objects are passed by "copy of a reference".

Specifically, when you pass an object (or array) you are (invisibly) passing a reference to that object, and it is possible to modify the contents of that object, but if you attempt to overwrite the reference it will not affect the copy of the reference held by the caller - i.e. the reference itself is passed by value:

function replace(ref) {
    ref = {};           // this code does _not_ affect the object passed
}

function update(ref) {
    ref.key = 'newvalue';  // this code _does_ affect the _contents_ of the object
}

var a = { key: 'value' };
replace(a);  // a still has its original value - it's unmodfied
update(a);   // the _contents_ of 'a' are changed

Think of it like this:

Whenever you create an object in ECMAscript, this object is formed in a mystique ECMAscript universal place where no man will ever be able to get. All you get back is a reference to that object in this mystique place.

var obj = { };

Even obj is only a reference to the object (which is located in that special wonderful place) and hence, you can only pass this reference around. Effectively, any piece of code which accesses obj will modify the object which is far, far away.


My two cents.... It's irrelevant whether JavaScript passes parameters by reference or value. What really matters is assignment vs. mutation.

I wrote a longer, more detailed explanation in this link.

When you pass anything (whether that be an object or a primitive), all JavaScript does is assign a new variable while inside the function... just like using the equal sign (=).

How that parameter behaves inside the function is exactly the same as it would behave if you just assigned a new variable using the equal sign... Take these simple examples.

var myString = 'Test string 1';

// Assignment - A link to the same place as myString
var sameString = myString;

// If I change sameString, it will not modify myString,
// it just re-assigns it to a whole new string
sameString = 'New string';

console.log(myString); // Logs 'Test string 1';
console.log(sameString); // Logs 'New string';

If I were to pass myString as a parameter to a function, it behaves as if I simply assigned it to a new variable. Now, let's do the same thing, but with a function instead of a simple assignment

function myFunc(sameString) {

  // Reassignment... Again, it will not modify myString
  sameString = 'New string';
}

var myString = 'Test string 1';

// This behaves the same as if we said sameString = myString
myFunc(myString);

console.log(myString); // Again, logs 'Test string 1';

The only reason that you can modify objects when you pass them to a function is because you are not reassigning... Instead, objects can be changed or mutated.... Again, it works the same way.

var myObject = { name: 'Joe'; }

// Assignment - We simply link to the same object
var sameObject = myObject;

// This time, we can mutate it. So a change to myObject affects sameObject and visa versa
myObject.name = 'Jack';
console.log(sameObject.name); // Logs 'Jack'

sameObject.name = 'Jill';
console.log(myObject.name); // Logs 'Jill'

// If we re-assign it, the link is lost
sameObject = { name: 'Howard' };
console.log(myObject.name); // Logs 'Jill'

If I were to pass myObject as a parameter to a function, it behaves as if I simply assigned it to a new variable. Again, the same thing with the exact same behavior but with a function.

function myFunc(sameObject) {
  // We mutate the object, so the myObject gets the change too... just like before.
  sameObject.name = 'Jill';

  // But, if we re-assign it, the link is lost
  sameObject = {
    name: 'Howard'
  };
}

var myObject = {
  name: 'Joe'
};

// This behaves the same as if we said sameObject = myObject;
myFunc(myObject);
console.log(myObject.name); // Logs 'Jill'

Every time you pass a variable to a function, you are "assigning" to whatever the name of the parameter is, just like if you used the equal = sign.

Always remember that the equals sign = means assignment. And passing a parameter to a function also means assignment. They are the same and the two variables are connected in exactly the same way.

The only time that modifying a variable affects a different variable is when the underlying object is mutated.

There is no point in making a distinction between objects and primitives, because it works the same exact way as if you didn't have a function and just used the equal sign to assign to a new variable.

Tags:

Javascript

Pass By Reference

Pass By Value

Reference

Recent Posts