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noCommaOperator (since v1.0.0)

Diagnostic Category: lint/style/noCommaOperator

Sources:

Disallow comma operator.

The comma operator includes multiple expressions where only one is expected. It evaluates every operand from left to right and returns the value of the last operand. It frequently obscures side effects, and its use is often an accident.

The use of the comma operator in the initialization and update parts of a for is still allowed.

Examples

Section titled Examples

Invalid

Section titled Invalid
const foo = (doSomething(), 0);
style/noCommaOperator.js:1:27 lint/style/noCommaOperator ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ The comma operator is disallowed.
  
  > 1 │ const foo = (doSomething(), 0);
      │                           ^
    2 │ 
  
  ℹ Its use is often confusing and obscures side effects.
  

for (; doSomething(), !!test; ) {}
style/noCommaOperator.js:1:21 lint/style/noCommaOperator ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ The comma operator is disallowed.
  
  > 1 │ for (; doSomething(), !!test; ) {}
      │                     ^
    2 │ 
  
  ℹ Its use is often confusing and obscures side effects.
  

// Use a semicolon instead.
let a, b;
a = 1, b = 2;
style/noCommaOperator.js:3:6 lint/style/noCommaOperator ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ The comma operator is disallowed.
  
    1 │ // Use a semicolon instead.
    2 │ let a, b;
  > 3 │ a = 1, b = 2;
      │      ^
    4 │ 
  
  ℹ Its use is often confusing and obscures side effects.

Valid

Section titled Valid
for(a = 0, b = 0; (a + b) < 10; a++, b += 2) {}
Section titled Related links