Testing for Strict Mode in JavaScript Using this

The this variable is an important concept in JavaScript and in many Object-Oriented languages. (Technically this is a keyword, but that’s a minor point in understanding it.) The simplest way to think of this is that it references the object whose method is currently being called. (The harder, but more accurate, way of thinking of this is as a representation of the execution context, but I find that definition makes a person blink a lot while simultaneously scratching his or her head.) The brilliant Peter-Paul Koch describes this as the “owner” of the current method, which also works.

To put this into a context (double pun), take the following JavaScript code, which defines a custom object, representing a rectangle:

var r = {
    side1: 10,
    side2: 20,
    isSquare: function() {
        return this.side1 == this.side2;
    }
};

That code defines an object with two attributes and one method. The method, called using the code r.isSquare(), returns a Boolean value indicating if the rectangle is also a square or not. Since the isSquare() method is defined as part of the r object, within that method, this refers to r.

Now take this code:

function returnThis() {
    return this;
}

In JavaScript running within the Web browser, any function defined outside of any object becomes a method of the global window object. Thus, in that code, this refers to the global window object.

Those two examples explain the simplest implications of the this keyword. However, as of ECMAScript 5, JavaScript now has a strict mode, triggered by adding the string use strict to a page or function:

function someFunction() {
    'use strict';
}

function returnThis() {
    'use strict';
    return this;
}

Taking this knowledge one step further, because this is undefined in strict mode in functions not associated with objects, you can use that fact to test whether or not your JavaScript is working in strict mode. To do so, just create a function not associated with any object and have it return a Boolean indicating whether or not this is undefined:

function isStrict() {
    return (typeof this == 'undefined');
}

function isStrict() {
    return !this;
}

var strict = isStrict();

var strict = (function() { return !this; }());