Why is this an issue?

A common code smell that can hinder the clarity of source code is making assignments within sub-expressions. This practice involves assigning a value to a variable inside a larger expression, such as within a loop or a conditional statement.

This practice essentially gives a side-effect to a larger expression, thus making it less readable. This often leads to confusion and potential errors.

Moreover, using chained assignments in declarations is also dangerous because one may accidentally create global variables. Consider the following code snippet: let x = y = 1;. If y is not declared, it will be hoisted as global.

Exceptions

The rule does not raise issues for the following patterns:

• chained assignments: a = b = c = 0;
• relational assignments: (a = 0) != b
• sequential assignments: a = 0, b = 1, c = 2
• assignments in lambda body: () => a = 0
• conditional assignment idiom: a || (a = 0)
• assignments in (do-)while conditions: while (a = 0);

How to fix it

Making assignments within sub-expressions can hinder the clarity of source code.

This practice essentially gives a side-effect to a larger expression, thus making it less readable. This often leads to confusion and potential errors.

Extracting assignments into separate statements is encouraged to keep the code clear and straightforward.

Code examples

Noncompliant code example

if (val = value() && check()) { // Noncompliant
  // ...
}

Compliant solution

val = value();
if (val && check()) {
  // ...
}

Resources

• CWE - CWE-481 - Assigning instead of Comparing