JavaScript Error Handling

JavaScript Errors – SyntaxError: test for equality mistyped as assignment?

Making our way through the wonders of the JavaScript Error Handling series, today we’ll explore the Equality Mistyped as Assignment SyntaxError. The Equality Mistyped as Assignment error is thrown when an attempt is made to declare an assignment of a variable, where the parsing engine believes the intention was an equality test instead.

In this article we’ll look closely at the Equality Mistyped as Assignment error, seeing where it fits within the JavaScript Exception hierarchy, and we’ll also go over a simple code example to illustrate how Equality Mistyped as Assignment errors might be typically thrown in real code. Let’s get to it!

The Technical Rundown

  • All JavaScript error objects are descendants of the Error object, or an inherited object therein.
  • The SyntaxError object is inherited from the Error object.
  • The Equality Mistyped as Assignment error is a specific type of SyntaxError object.

When Should You Use It?

Since the Equality Mistyped as Assignment error deals with the concepts of equality and assignments, we should briefly cover both before diving into the error itself.

As with many other programming languages, JavaScript’s equality operator is simply a pair of equals signs (==) separating the two objects to be compared. If the two objects are not the same type, the engine will first attempt to convert them to matching types (such as converting a string to a number), before comparing their equivalence. In the case where both items to be compared are objects, the comparison doesn’t check the objects’ values (since it’s unaware of what those are), and instead simply checks the in-memory reference (memory address) of both objects, to see if they’re equivalent.

As discussed in the official documentation, these differences in equality operators can be seen below:

The assignment operator found in JavaScript is even more common across other languages and is typically a single equals sign (=) separating the variable and the value. JavaScript also allows assignment operators to be chained onto one another, which forces the engine to execute (and assign) via “outside-in” order, starting with the outermost assignment and working inward:

Armed with the basics of assignment and equality operators, we can now take a closer look at the Equality Mistyped as Assignment error. As mentioned in the introduction, the basic problem when an Equality Mistyped as Assignment error is thrown is that the JavaScript parser notices an assignment operator in a location it expects that an equality operator might have been intended.

Before we see an example of this it’s critical to note that, as with a few other errors we’ve explored in this series, the Equality Mistyped as Assignment error can only be thrown when strict mode is enabled in JavaScript. In short, strict mode is a toggled directive that forces JavaScript to behave in a slightly altered manner, usually by opting into more secure limitations placed upon the code, and thereby reducing the possibility of executing unintended code or performing unintended functionality. Since strict mode can heighten overall security, it is considered a requirement in certain coding situations, and in such cases, it’s entirely possible to throw an Equality Mistyped as Assignment error.

In most cases, strict mode is enabled by including the 'use strict'; directive at the start (and within the same scope) of the code you wish to execute via strict limitations:

Since strict mode remembers its own local scope of execution, most uses of strict mode are performed inside an immediately invoked function expression (IIFE), which is a syntactic method of defining a function, by surrounding it with parentheses, which effectively tells the JavaScript engine that this function should be treated as a grouped entity and executed (invoked) immediately. Thus, our example code is contained within an IIFE, and includes an assignment operator inside the if statement clause:

While very basic, this code performs an assignment within the if statement on line 7, which is interpreted as an intended equality operator instead. This causes an Equality Mistyped as Assignment error to be thrown in Firefox:

As we’ve discussed many times before, different JavaScript engines (from different browsers) handle parsing and exceptions differently. In this case, the latest version of Chrome does not recognize this as an issue, and thus ignores the “error” entirely, simply performing the name = 'Jane' assignment, as our code told it to, and outputting the new result. I leave it up to you to decide if this is an issue or not, but it’s just something to be aware of when developing and testing on different platforms.

It’s also worth noting that, just as with other SyntaxErrors, the Equality Mistyped as Assignment error is difficult to catchin a typical try-catch code block. Here we try surrounding our IIFE example snippet in a try-catch block, but the JavaScript parser recognizes (and reacts to) the SyntaxError before it has a chance to notice (and thus execute) the surrounding try-catch code:

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