In JavaScript, most operators can be applied to operands of arbitrary types; at runtime, the operands will be implicitly converted to the appropriate type. For instance, the expression p in obj checks whether the object obj contains a property whose name equals the string that p evaluates to. If p does not evaluate to a string or o does not evaluate to an object, implicit conversions are performed before the check is carried out.

In many cases, however, these implicit conversions result from a typo or a misunderstanding of operator precedence rules. Even if the conversions are intentional, relying on them makes the code hard to understand.

Inspect the expression carefully to check whether the operands have been mistyped, and correct them if this is the case. If the conversions are intentional, consider replacing them by explicit conversions to clarify the meaning of the code.

The following code intends to check whether object obj does not contain a property of the name stored in variable member:

However, this test is ineffective as written: the operator ! binds more tightly than in, so it is applied first. Applying ! to a non-empty string yields false, so the in operator actually ends up checking whether obj contains a property called "false".

To fix this, parentheses should be introduced as follows:

As an example of the intentional use of implicit conversions, consider the following function for comparing two numbers x and y. It returns 1 if x>y, -1 if x<y, and 0 if they are equal.

It would be much clearer to write this out directly:

At the very least, the Boolean comparison results should be explicitly converted to numbers:

Ecma International, ECMAScript Language Definition, 5.1 Edition, Section 9. ECMA, 2011.