Suspicious type coercion should be avoided

• BAD_TYPE_COERCION
• Error
• Medium
• cwe

This rule applies when suspicious type coercion occurs.

JavaScript utilizes implicit type conversion widely, so a programmer needs to check type conversion with caution.

The following is suspicious implicit type conversion for X:

1. X is converted to string.

   • X + string (X is undefined or symbol.)

2. X is converted to number.

   • X + undefined/null/boolean/number/symbol (X is undefined, null, or symbol.)
   • X @ <All types> (X is undefined/null/non-numeric string/symbol/object. @ means -, *, /, %, **.)
   • X @ Y (X is undefined/null/boolean/non-numeric string/symbol except for the case where X and Y are both strings. @ means <, <=, >, >=.)
   • X @ number (X is object. @ means <, <=, >, >=.)
   • X & <All types> (X is undefined/null/boolean/non-numeric string/symbol/object.)
   • X | boolean (X is boolean.)
   • X | Y (X is symbol.)
   • X @ Y (X is boolean and Y is number/string/object. Or X is non-numeric string and Y is boolean/number. Or X is object and Y is boolean/number. @ means ==, !=.)
   • @ X (X is non-numeric string. @ means +, -.)

3. X is converted to boolean. (X is a primitive wrapper object.)

   • if (X) …
   • X && …
   • X || …
   • X ? … : …
   • !X

For your reference, JavaScript converts a type like the following:

1. undefined is converted to string: "undefined"
2. undefined is converted to number: NaN
3. null is converted to string: "null"
4. null is converted to number: 0
5. boolean is converted to number: false is 0, true is 1.
6. Empty string is converted to number: 0
7. Neither empty nor numeric string is converted to number: NaN
8. Symbol is converted to string or number: a TypeError exception is thrown.
9. Primitive wrapper object is converted to boolean: true

Noncompliant Code Example

function example1() {
    var cssString = "";
    var backPosition;
    if (animatedBackground.length === 3) {
        cssString += "backgroundPosition: " + backPosition + "px; "; // BAD_TYPE_COERCION alarm: Expression 'backPosition' has an undefined value and type-coerced to string type.
    }
    return cssString;
}

function example2() {
    var mod = $index & 1;
    if (mod !== old$index & 1) { // BAD_TYPE_COERCION alarm: Expression 'mod !== old$index' has boolean type and type-coerced to number type.
        if (mod === selector) {
            addClass(scope.$eval(attr[name]));
        } else {
            removeClass(scope.$eval(attr[name]));
        }
    }
}

Compliant Code Example

function example1() {
    var cssString = "";
    var backPosition;
    if (animatedBackground.length === 3) {
        backPosition = 0; // Assign a valid value.
        cssString += "backgroundPosition: " + backPosition + "px; ";
    }
    return cssString;
}

function example2() {
    var mod = $index & 1;
    if (mod !== (old$index & 1)) { // Use parentheses.
        if (mod === selector) {
            addClass(scope.$eval(attr[name]));
        } else {
            removeClass(scope.$eval(attr[name]));
        }
    }
}

Version

This rule was introduced in DeepScan 1.0.0-alpha.

See

• CWE-843