no-cond-assign

Disallow assignment operators in conditional expressions

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
}
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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
let x;
if (x = 0) {
    const b = 1;
}

// Practical example that is similar to an error
const setHeight = function (someNode) {
    do {
        someNode.height = "100px";
    } while (someNode = someNode.parentNode);
}
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Examples of correct code for this rule with the default "except-parens" option:

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

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

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

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

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

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

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

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

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

// Practical example that wraps the assignment and tests for 'null'
const heightSetter = function (someNode) {
    do {
        someNode.height = "100px";
    } while ((someNode = someNode.parentNode) !== null);
}
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Examples of correct code for this rule with the "always" option:

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

// Assignment replaced by comparison
let x;
if (x === 0) {
    const b = 1;
}
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Related Rules

• no-extra-parens

Version

This rule was introduced in ESLint v0.0.9.

Resources

• Rule source
• Tests source