Using the same value on either side of a binary operator is almost always a mistake. In the case of logical operators, it is either a copy/paste error and therefore a bug, or it is simply wasted code, and should be simplified. In the case of bitwise operators and most binary mathematical operators, having the same value on both sides of an operator yields predictable results, and should be simplified.

This rule ignores `*`, `+`, and `=`.

## Noncompliant Code Example

    if ( a == a ) { // always true
      doZ();
    }
    if ( a != a ) { // always false
      doY();
    }
    if ( a == b && a == b ) { // if the first one is true, the second one is too
      doX();
    }
    if ( a == b || a == b ) { // if the first one is true, the second one is too
      doW();
    }

    var j = 5 / 5; //always 1
    var k = 5 - 5; //always 0

## Compliant Solution

    doZ();

    if ( a == b ) {
      doX();
    }
    if ( a == b ) {
      doW();
    }

    var j = 1;
    var k = 0;

## Exceptions

The specific case of testing one variable against itself is a valid test for `NaN` and is therefore ignored.

Similarly, left-shifting 1 onto 1 is common in the construction of bit masks, and is ignored.

    if(f !== f) { // test for NaN value
      console.log("f is NaN");
    }

    var i = 1 << 1; // Compliant
    var j = a << a; // Noncompliant

## See

- CERT, MSC12-C - Detect and remove code that has no effect
- CERT, MSC12-CPP - Detect and remove code that has no effect
- Rule {rule:javascript:S1656} - Implements a check on `=`.