A dead store happens when a local variable is assigned a value that is not read by any subsequent instruction. Calculating or retrieving a value only to then overwrite it or throw it away, could indicate a serious error in the code. Even if it’s not an error, it is at best a waste of resources. Therefore all calculated values should be used.

Noncompliant Code Example

i = a + b; // Noncompliant; calculation result not used before value is overwritten
i = compute();

Compliant Solution

i = a + b;
i += compute();

Exceptions

• This rule ignores initializations to -1, 0, 1, undefined, [], {}, true, false and "".
• Variables that start with an underscore (e.g. '_unused') are ignored.
• Assignment of null is ignored because it is sometimes used to help garbage collection
• Increment and decrement expressions are ignored because they are often used idiomatically instead of x+1
• This rule also ignores variables declared with object destructuring using rest syntax (used to exclude some properties from object):

let {a, b, ...rest} = obj; // 'a' and 'b' are ok
doSomething(rest);

let [x1, x2, x3] = arr;    // but 'x1' is noncompliant, as omitting syntax can be used: "let [, x2, x3] = arr;"
doSomething(x2, x3);

See

• MITRE, CWE-563 - Assignment to Variable without Use ('Unused Variable')