# Error Prone

Rules to detect constructs that are either broken, extremely confusing or prone to runtime errors.

## AvoidTrailingComma

Since: PMD 5.1

Priority: High (1)

This rule helps improve code portability due to differences in browser treatment of trailing commas in object or array literals.

This rule is defined by the following XPath expression:

    //ObjectLiteral[$allowObjectLiteral = false() and @TrailingComma = true()]
    |
    //ArrayLiteral[$allowArrayLiteral = false() and @TrailingComma = true()]

Example(s):

    function(arg) {
        var obj1 = { a : 1 };   // Ok
        var arr1 = [ 1, 2 ];    // Ok

        var obj2 = { a : 1, };  // Syntax error in some browsers!
        var arr2 = [ 1, 2, ];   // Length 2 or 3 depending on the browser!
    }

This rule has the following properties:

Name | Default Value | Description | Multivalued
--- | --- | --- | ---
allowObjectLiteral | false | Allow a trailing comma within an object literal | no
allowArrayLiteral | false | Allow a trailing comma within an array literal | no

Use this rule with the default properties by just referencing it:

    <rule ref="category/ecmascript/errorprone.xml/AvoidTrailingComma" />

Use this rule and customize it:

    <rule ref="category/ecmascript/errorprone.xml/AvoidTrailingComma">
        <properties>
            <property name="allowObjectLiteral" value="false" />
            <property name="allowArrayLiteral" value="false" />
        </properties>
    </rule>

## EqualComparison

Since: PMD 5.0

Priority: Medium (3)

Using == in condition may lead to unexpected results, as the variables are automatically casted to be of the same type. The === operator avoids the casting.

This rule is defined by the following XPath expression:

    //InfixExpression[
      (@Image = "==" or @Image = "!=")
      and
      (KeywordLiteral[@Image='true' or @Image = 'false'] or NumberLiteral)
    ]

Example(s):

    // Ok
    if (someVar === true) {
      ...
    }
    // Ok
    if (someVar !== 3) {
      ...
    }
    // Bad
    if (someVar == true) {
      ...
    }
    // Bad
    if (someVar != 3) {
      ...
    }

Use this rule by referencing it:

    <rule ref="category/ecmascript/errorprone.xml/EqualComparison" />

## InnaccurateNumericLiteral

Since: PMD 5.0

Priority: Medium High (2)

The numeric literal will have a different value at runtime, which can happen if you provide too much precision in a floating point number. This may result in numeric calculations being in error.

This rule is defined by the following XPath expression:

    //NumberLiteral[@NormalizedImage != string(@Number)]

Example(s):

    var a = 9; // Ok
    var b = 999999999999999; // Ok
    var c = 999999999999999999999; // Not good
    var w = 1.12e-4; // Ok
    var x = 1.12; // Ok
    var y = 1.1234567890123; // Ok
    var z = 1.12345678901234567; // Not good

Use this rule by referencing it:

    <rule ref="category/ecmascript/errorprone.xml/InnaccurateNumericLiteral" />