Comparing two identical expressions typically indicates a mistake such as a missing this qualifier or a misspelled variable name. The only case where such a comparison makes semantic sense is when determining whether a value is NaN, since NaN does not compare as equal to itself. But even in this case clearer alternatives are available.

Carefully inspect the comparison to determine whether it is a symptom of a bug. If the comparison is used to check for NaN, consider using the isNaN function from the standard library instead.

In the example below, the method Rectangle.prototype.contains is intended to check whether a point (x, y) lies inside a rectangle given by its origin (this.x, this.y), its width this.width, and its height this.height.

Note, however, that on line 11 the programmer forgot to qualify this.y, thus ending up comparing the argument y against itself. The comparison should be fixed accordingly:

A common use of self-comparison is to detect NaN values, which are the only kind of values that are not considered equal to themselves:

function eq(x, y) { // check if x is NaN if (x !== x) { // consider NaN to be equal to itself return y !== y; } return x === y; }

In cases like this one, it is clearer to use the isNaN function from the standard library:

function eq(x, y) { // check if x is NaN if (isNaN(x)) { // consider NaN to be equal to itself return isNaN(y); } return x === y; }

If you do not want to rely on isNaN being defined, you can provide your own implementation: self-comparisons in functions named isNaN (regardless of capitalization) are treated specially and will not be flagged.

Mozilla Developer Network: NaN.