Cover by Nicholas C. Zakas

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Chapter 8. Avoid Null Comparisons

A common yet still problematic pattern in JavaScript is testing a variable against null, presumably to determine whether the variable has been filled in with an appropriate value. For example:

var Controller = {
    process: function(items) {
        if (items !== null) {     // Bad
            items.sort();
            items.forEach(function(item) {
                // do something
            });
        }
    }
};

Here, the process() method is clearly expecting that items will be an array, as indicated by the use of sort() and forEach(). The intention of this code is clear: don’t continue processing unless the items argument contains an array. The problem with this approach is that the comparison against null doesn’t actually prevent future errors. The value of items could be 1, or a string, or some random object. All of these are technically not equal to null and would therefore cause the process() method to fail once sort() executes.

Comparing a variable against only null typically doesn’t give you enough information about the value to determine whether it’s safe to proceed. Fortunately, JavaScript gives you a number of ways to determine the true value of a variable.

Detecting Primitive Values

There are five primitive types in JavaScript: string, number, boolean, null, and undefined. If you are expecting a value to be a string, number, boolean, or undefined, then the typeof operator is your best option. The typeof operator works on a variable and returns a string indicating the type of value:

  • For strings, typeof returns “string.” ...

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