Javascript: Pass By Reference Or By Value?

Is Javascript pass by reference or pass by value? In fact, it’s a little bit of both. Primitives: null, undefined, Number, String, & Boolean are pass by value, and Objects: Array, Function, & Objects are pass by reference. Unsure what we mean? Let’s check some examples with comments to see exactly!

// Primitives won't get changed, because we copy (pass) the value
// so the underlying value won't get changed
let a = 1;
let b = a;

a = 5;

console.log(a); // 5
console.log(b); // 1

// we copy by reference meaning when we assign b to a we copy the location in memory
// meaning the underlying object will get changed
let a = {};
let b = a;

a.foo = 'bar';

console.log(a); // {foo: "bar"}
console.log(b); // {foo: "bar"}

// same is true for arrays, the underlying array will get changed
let z = [];
let x = z;

z.push('apple');

console.log(z); // ["apple"]
console.log(x); // ["apple"]

Javascript functions also behave similarly:

// functions can change the object without even returning the object because
// it is changing the same referenced object
function changeGrade(student) {
    student.math = 'A';
}

let student = {
    math: 'F',
    history: 'B'
};

changeGrade(student);
console.log(student); // {math: "A", history: "B"}

// A boolean is a primitive so it is passed by value so will not get 
// changed when passed to a function
function primitiveUnchanged(didPass) {
    didPass = true;
}

let didPass = false; 
primitiveUnchanged(didPass);

console.log(didPass); // false

A couple of good overviews can be found on:

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#advanced #javascript #objects