js/no-mixed-operators

Enclosing complex expressions by parentheses clarifies the developer's intention, which makes the code more readable. This rule warns when different operators are used consecutively without parentheses in an expression.

var foo = a && b || c || d;    /*BAD: Unexpected mix of '&&' and '||'.*/
var foo = (a && b) || c || d;  /*GOOD*/
var foo = a && (b || c || d);  /*GOOD*/

Note: It is expected for this rule to emit one error for each mixed operator in a pair. As a result, for each two consecutive mixed operators used, a distinct error will be displayed, pointing to where the specific operator that breaks the rule is used:

var foo = a && b || c || d;

will generate

1:13  Unexpected mix of '&&' and '||'. (no-mixed-operators)
1:18  Unexpected mix of '&&' and '||'. (no-mixed-operators)

Rule Details

This rule checks BinaryExpression, LogicalExpression and ConditionalExpression.

This rule may conflict with no-extra-parens rule. If you use both this and no-extra-parens rule together, you need to use the nestedBinaryExpressions option of no-extra-parens rule.

Examples of incorrect code for this rule:

::: incorrect

/*eslint no-mixed-operators: "error"*/

var foo = a && b < 0 || c > 0 || d + 1 === 0;
var foo = a + b * c;

:::

Examples of correct code for this rule:

::: correct

/*eslint no-mixed-operators: "error"*/

var foo = a || b || c;
var foo = a && b && c;
var foo = (a && b < 0) || c > 0 || d + 1 === 0;
var foo = a && (b < 0 || c > 0 || d + 1 === 0);
var foo = a + (b * c);
var foo = (a + b) * c;

:::

Options

{
    "no-mixed-operators": [
        "error",
        {
            "groups": [
                ["+", "-", "*", "/", "%", "**"],
                ["&", "|", "^", "~", "<<", ">>", ">>>"],
                ["==", "!=", "===", "!==", ">", ">=", "<", "<="],
                ["&&", "||"],
                ["in", "instanceof"]
            ],
            "allowSamePrecedence": true
        }
    ]
}

This rule has 2 options.

• groups (string[][]) - specifies operator groups to be checked. The groups option is a list of groups, and a group is a list of binary operators. Default operator groups are defined as arithmetic, bitwise, comparison, logical, and relational operators.
  Note: Coalesce operator ("??") and Ternary operator ("?:") are not part of any group in the default configuration and therefore are allowed to be mixed with all other operators.

• allowSamePrecedence (boolean) - specifies whether to allow mixed operators if they are of equal precedence. Default is true.

groups

The following operators can be used in groups option:

• Arithmetic Operators: "+", "-", "*", "/", "%", "**"
• Bitwise Operators: "&", "|", "^", "~", "<<", ">>", ">>>"
• Comparison Operators: "==", "!=", "===", "!==", ">", ">=", "<", "<="
• Logical Operators: "&&", "||"
• Relational Operators: "in", "instanceof"
• Coalesce Operator: "??"
• Ternary Operator: "?:"

The rule checks each configured group separately - warnings or errors are only triggered if operators within the same group are mixed (without using parentheses). Inter-group usage is not checked. Now, consider the following group configuration: {"groups": [["&", "|", "^", "~", "<<", ">>", ">>>"], ["&&", "||"]]}. There are 2 groups specified in this configuration: bitwise operators and logical operators. In this case, this rule checks if bitwise operators are mixed with bitwise operators, and if logical operators are mixed with logical operators, but ignores all other operators.

Examples of incorrect code for this rule with {"groups": [["&", "|", "^", "~", "<<", ">>", ">>>"], ["&&", "||"]]} option:

::: incorrect

/*eslint no-mixed-operators: ["error", {"groups": [["&", "|", "^", "~", "<<", ">>", ">>>"], ["&&", "||"]]}]*/

var foo = a && b < 0 || c > 0 || d + 1 === 0;
var foo = a & b | c;

:::

::: incorrect

/*eslint no-mixed-operators: ["error", {"groups": [["&&", "||", "?:"]]}]*/

var foo = a || b ? c : d;

var bar = a ? b || c : d;

var baz = a ? b : c || d;

:::

Examples of correct code for this rule with {"groups": [["&", "|", "^", "~", "<<", ">>", ">>>"], ["&&", "||"]]} option:

::: correct

/*eslint no-mixed-operators: ["error", {"groups": [["&", "|", "^", "~", "<<", ">>", ">>>"], ["&&", "||"]]}]*/

var foo = a || b > 0 || c + 1 === 0;
var foo = a && b > 0 && c + 1 === 0;
var foo = (a && b < 0) || c > 0 || d + 1 === 0;
var foo = a && (b < 0 ||  c > 0 || d + 1 === 0);
var foo = (a & b) | c;
var foo = a & (b | c);
var foo = a + b * c;
var foo = a + (b * c);
var foo = (a + b) * c;

:::

::: correct

/*eslint no-mixed-operators: ["error", {"groups": [["&&", "||", "?:"]]}]*/

var foo = (a || b) ? c : d;
var foo = a || (b ? c : d);

var bar = a ? (b || c) : d;

var baz = a ? b : (c || d);
var baz = (a ? b : c) || d;

:::

allowSamePrecedence

Examples of correct code for this rule with {"allowSamePrecedence": true} option:

::: correct

/*eslint no-mixed-operators: ["error", {"allowSamePrecedence": true}]*/

// + and - belong to the same default group; they have the same precedence.
var foo = a + b - c;

:::

Examples of incorrect code for this rule with {"allowSamePrecedence": false} option:

::: incorrect

/*eslint no-mixed-operators: ["error", {"allowSamePrecedence": false}]*/

// + and - belong to the same default group; they have the same precedence.
var foo = a + b - c;

:::

Examples of correct code for this rule with {"allowSamePrecedence": false} option:

::: correct

/*eslint no-mixed-operators: ["error", {"allowSamePrecedence": false}]*/

// + and - belong to the same default group; they have the same precedence.
var foo = (a + b) - c;

:::

When Not To Use It

If you don't want to be notified about mixed operators, then it's safe to disable this rule.