Assignment inside a Condition

It’s very common in PHP to see code written like this:

if($summary = get_post_summary())
{
    return $summary;
}

What I’m referring to is assignment inside a condition. Cunningly enough, it can be done in JavaScript too, although it’s far less common:

if(summary = document.getElementById("post-summary"))
{
    return summary.innerHTML;
}

At first glance you’d be forgiven for thinking that’s a typo! Indeed, some debuggers will flag that with a warning, asking if you meant a test for equality (==) and mistyped it as assignment (=).

But it’s not a mistake—the key to understanding it is to understand two things:

First, assignment returns a value (the value you assigned). Second and most importantly, the value it returns evaluates to true or false, and ultimately determines whether the condition passes.

Now, in the world of DOM evaluation and traversal, this technique is a safe and predictable one because DOM nodes either exist or they’re null—and null is required to evaluate to false. So in the code example above, the condition will evaluate to true if the "#post-summary" element exists, or false if it doesn’t.

Equally, you can use the same trick to iterate up an unknown hierarchy, traversing through successive parents using while(). This example builds an array of every node name between an event target and the #document:

var names = [], node = e ? e.target : event.srcElement;
do
{
    names.push(node.nodeName);
}
while(node = node.parentNode)

But elsewhere in JavaScript, you could find yourself in much less reliable territory, because how true or how false many values turn out to be is not at all intuitive.

Both positive and negative numbers, for example, evaluate to true except zero and NaN. And bizarrely, an object created with the Boolean constructor always evaluates to true, even if it was created as new Boolean(false)!

So be warned! Syntax like this is not for the fainthearted; nor should it be used in a cavalier way, with overreliance on type conversion and the casting of arbitrary values. But used well in suitable circumstances, it can simplify many conditions to produce faster and leaner code.

Thumbnail credit: sbwoodside

Frequently Asked Questions (FAQs) about Assignment Inside a Condition

What is an assignment inside a condition in programming?

An assignment inside a condition refers to the practice of assigning a value to a variable within a conditional statement such as an ‘if’ statement. This is a common practice in many programming languages including JavaScript, C++, and Python. It allows for more concise code as the assignment and the condition check can be done in a single line. However, it can also lead to confusion and potential bugs if not used carefully, as the assignment operation might be mistaken for a comparison operation.

Why is it considered bad practice to perform assignments in conditional expressions?

Assignments in conditional expressions can lead to confusion and potential bugs. This is because the assignment operator (=) can easily be mistaken for the equality operator (==). As a result, a condition that was meant to be a comparison could inadvertently become an assignment, leading to unexpected behavior in the code. Additionally, assignments in conditions can make the code more difficult to read and understand, particularly for less experienced programmers.

How can I avoid assignments in conditional expressions?

To avoid assignments in conditional expressions, you can separate the assignment and the condition check into two separate lines of code. For example, instead of writing if (x = getValue()), you could write x = getValue(); if (x). This makes the code clearer and reduces the risk of confusion or bugs.

Are there any situations where assignments in conditions are acceptable or even beneficial?

While generally discouraged, there are situations where assignments in conditions can be beneficial. For example, in a loop where a value needs to be updated and checked in each iteration, an assignment in the condition can make the code more concise. However, this should be done with caution and the code should be clearly commented to avoid confusion.

What is the difference between the assignment operator and the equality operator?

The assignment operator (=) is used to assign a value to a variable. For example, x = 5 assigns the value 5 to the variable x. On the other hand, the equality operator (==) is used to compare two values. For example, if (x == 5) checks whether the value of x is equal to 5.

How does TypeScript handle assignments in conditions?

TypeScript, like JavaScript, allows assignments in conditions. However, TypeScript has stricter type checking which can help catch potential errors caused by assignments in conditions. For example, if you try to assign a string to a variable that is supposed to be a number inside a condition, TypeScript will give a compile-time error.

What are some common bugs caused by assignments in conditions?

One common bug caused by assignments in conditions is an unintended assignment when a comparison was intended. For example, if (x = 5) will always be true because it assigns 5 to x, rather than checking if x is equal to 5. This can lead to unexpected behavior in the code.

How can I debug issues caused by assignments in conditions?

Debugging issues caused by assignments in conditions can be tricky because the code might not give any errors. One approach is to carefully check all conditional statements to ensure that they are using the correct operators. Using a linter or a static code analysis tool can also help catch these issues.

Can assignments in conditions be used in all programming languages?

Not all programming languages allow assignments in conditions. For example, Python does not allow assignments in conditions and will give a syntax error if you try to do so. Always check the syntax rules of the programming language you are using.

Are there any alternatives to assignments in conditions?

Yes, there are alternatives to assignments in conditions. One common alternative is to use a temporary variable to hold the value that needs to be assigned and checked. This can make the code clearer and easier to understand. Another alternative is to use a function that returns a value and then check that value in the condition.