Scope Safe Constructors in Javascript

Last modified: 
Thursday, April 9th, 2015

Scope-safe constructors ensure Javascript objects created without the ''new'' keyword will still behave as expected as regards scoping.

function MyClass(someValue) {
    if (this instanceof MyClass) {
        this.someValue = someValue;
    else {
        return new MyClass(someValue);

If you don't use scope-safe constructors and unwittingly instantiate an object without the new keyword, this will be in the global scope.

function MyClass(x) {
    this.x = x; 

// Here AnObject has been instantiated without the "new" keyword.
var AnObject = MyClass(5);

// It seems to work as expected, but...
console.log(AnObject.x); // 5

// ...what happens set a value to x while in the global scope?
x = 12;

// The value of AnObject.x is updated to 12, thus encapsulation is broken!
console.log(AnObject.x); // 12

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