disallow assignment operators in conditional statements (no-cond-assign)

In conditional statements, it is very easy to mistype a comparison operator (such as ==) as an assignment operator (such as =). For example:

// Check the user's job title
if (user.jobTitle = "manager") {
    // user.jobTitle is now incorrect
}

There are valid reasons to use assignment operators in conditional statements. However, it can be difficult to tell whether a specific assignment was intentional.

Rule Details

This rule disallows ambiguous assignment operators in test conditions of if, for, while, and do...while statements.

Options

This rule has a string option:

• "except-parens" (default) allows assignments in test conditions only if they are enclosed in parentheses (for example, to allow reassigning a variable in the test of a while or do...while loop)
• "always" disallows all assignments in test conditions

except-parens

Examples of incorrect code for this rule with the default "except-parens" option:

/*eslint no-cond-assign: "error"*/

// Unintentional assignment
var x;
if (x = 0) {
    var b = 1;
}

// Practical example that is similar to an error
function setHeight(someNode) {
    "use strict";
    do {
        someNode.height = "100px";
    } while (someNode = someNode.parentNode);
}

Examples of correct code for this rule with the default "except-parens" option:

/*eslint no-cond-assign: "error"*/

// Assignment replaced by comparison
var x;
if (x === 0) {
    var b = 1;
}

// Practical example that wraps the assignment in parentheses
function setHeight(someNode) {
    "use strict";
    do {
        someNode.height = "100px";
    } while ((someNode = someNode.parentNode));
}

// Practical example that wraps the assignment and tests for 'null'
function setHeight(someNode) {
    "use strict";
    do {
        someNode.height = "100px";
    } while ((someNode = someNode.parentNode) !== null);
}

always

Examples of incorrect code for this rule with the "always" option:

/*eslint no-cond-assign: ["error", "always"]*/

// Unintentional assignment
var x;
if (x = 0) {
    var b = 1;
}

// Practical example that is similar to an error
function setHeight(someNode) {
    "use strict";
    do {
        someNode.height = "100px";
    } while (someNode = someNode.parentNode);
}

// Practical example that wraps the assignment in parentheses
function setHeight(someNode) {
    "use strict";
    do {
        someNode.height = "100px";
    } while ((someNode = someNode.parentNode));
}

// Practical example that wraps the assignment and tests for 'null'
function setHeight(someNode) {
    "use strict";
    do {
        someNode.height = "100px";
    } while ((someNode = someNode.parentNode) !== null);
}

Examples of correct code for this rule with the "always" option:

/*eslint no-cond-assign: ["error", "always"]*/

// Assignment replaced by comparison
var x;
if (x === 0) {
    var b = 1;
}

Related Rules

• [no-extra-parens](no-extra-parens.md) Source: http://eslint.org/docs/rules/

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 132.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76