Self-Assignment

ID

javascript.self_assignment

Severity

high

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

JavaScript

Tags

CWE:480, reliability, suspicious-comparison

Description

Reports an assignment whose two sides are the same expression: x = x, this.count = this.count, a[i] = a[i]. The statement has no effect at run time.

The two sides are compared over the parsed tokens, so comments, whitespace and line breaks are ignored while a difference in operand, property or index is not. Compound assignments are a different statement and are never reported — x += x doubles x. Either side is also left alone when it contains a call or an increment, since those make two textually identical sides evaluate differently.

Rationale

// Bad
function Counter(count) {
  this.count = this.count;   // meant: this.count = count
}

function copy(src, dst, i) {
  dst[i] = dst[i];           // meant: dst[i] = src[i]
}

  • The statement is either dead weight or, far more often, a typo for the assignment that was meant — a missing this. on one side, or an operand left unchanged after a copy-paste.

  • Nothing fails: the field keeps its previous value (frequently undefined) and the defect surfaces much later, far from its cause.

  • A self-assignment used deliberately as a "mark this parameter as used" trick has no effect either; a lint suppression or an explicit void x says so plainly.

Remediation

Assign the value that was meant, or delete the statement.

// Good
function Counter(count) {
  this.count = count;
}

function copy(src, dst, i) {
  dst[i] = src[i];
}

References

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