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BUG001-assignment-in-condition — Assignment in condition

PropertyValue
DimensionMaintainability
SeverityMedium
ConfidenceHigh
CWECWE-480
LanguagesJavaScript, TypeScript

What it detects​

An assignment expression (=, +=, -=, *=, /=, etc.) that appears in the test position of a conditional statement or expression:

  • if (x = 1) { … } — almost always a typo for if (x == 1)
  • while (x = nextChunk()) { … } — assignment in loop guard
  • do { … } while (x = next())
  • for (let i = 0; x = step(); i++) — the test slot only; the i = 0 init is a VariableDeclaration and is not flagged
  • Ternary test: (x = 1) ? a : b

Carve-out​

Following ESLint's no-cond-assign "except-parens" default, an assignment wrapped in an extra pair of parentheses is not flagged. The idiom if ((x = getValue())) is the documented "I really mean it" pattern:

// NOT flagged — intentional double-paren carve-out:
if ((x = getValue())) {
    use(x);
}

Why it matters​

CWE-480 (Use of Incorrect Operator) documents the = vs == confusion as a source of real bugs. The code compiles and runs, but the condition always evaluates to the assigned value rather than a boolean comparison, silently changing program logic.

// Bug: assigns 0 (falsy) — body never executes
if (status = 0) { handleSuccess(); }

// Intended:
if (status === 0) { handleSuccess(); }

Examples — flagged​

// Simple assignment instead of comparison
if (x = 1) {
    doSomething();
}

// Compound assignment in while guard
while (buf = readChunk()) {
    process(buf);
}

// do-while
do {
    process(x);
} while (x = next());

// for — test slot only
for (let i = 0; x = step(); i++) {
    // let i = 0  →  init, NOT flagged
    // x = step() →  test slot, flagged
    // i++        →  update, NOT flagged
}

// TypeScript cast does not suppress the finding
if (x = getValue() as number) {
    use(x);
}

Examples — not flagged​

// Equality / strict equality — correct
if (x == 1) { … }
if (x === 1) { … }

// Assignment outside condition
let x = 1;
if (x) { … }

// Intentional assignment with extra parens (ESLint except-parens carve-out)
if ((x = getValue())) { use(x); }

// Assignment in ternary branch position (not the test)
cond ? (a = b) : c;

Fix guidance​

Replace the assignment with the intended comparison:

// Before (bug):
if (result = compute()) { … }

// After (fix — comparison):
if (result === compute()) { … }

// After (fix — intentional assignment, explicit style):
result = compute();
if (result) { … }

// After (alternative — intentional assignment with carve-out):
if ((result = compute())) { … }

Implementation​

References​