Skip to content
Biome
Docs Blog Playground

noAssignInExpressions (since v1.0.0)

Diagnostic Category: lint/suspicious/noAssignInExpressions

Sources:

Disallow assignments in expressions.

In expressions, it is common to mistype a comparison operator (such as ==) as an assignment operator (such as =). Moreover, the use of assignments in expressions is confusing. Indeed, expressions are often considered as side-effect free.

Examples

Section titled Examples

Invalid

Section titled Invalid
let a, b;
a = (b = 1) + 1;
suspicious/noAssignInExpressions.js:2:6 lint/suspicious/noAssignInExpressions ━━━━━━━━━━━━━━━━━━━━━━

  ✖ The assignment should not be in an expression.
  
    1 │ let a, b;
  > 2 │ a = (b = 1) + 1;
      │      ^^^^^
    3 │ 
  
  ℹ The use of assignments in expressions is confusing.
    Expressions are often considered as side-effect free.
  

let a;
if (a = 1) {
}
suspicious/noAssignInExpressions.js:2:5 lint/suspicious/noAssignInExpressions ━━━━━━━━━━━━━━━━━━━━━━

  ✖ The assignment should not be in an expression.
  
    1 │ let a;
  > 2 │ if (a = 1) {
      │     ^^^^^
    3 │ }
    4 │ 
  
  ℹ The use of assignments in expressions is confusing.
    Expressions are often considered as side-effect free.
  

function f(a) {
    return a = 1;
}
suspicious/noAssignInExpressions.js:2:12 lint/suspicious/noAssignInExpressions ━━━━━━━━━━━━━━━━━━━━━

  ✖ The assignment should not be in an expression.
  
    1 │ function f(a) {
  > 2 │     return a = 1;
      │            ^^^^^
    3 │ }
    4 │ 
  
  ℹ The use of assignments in expressions is confusing.
    Expressions are often considered as side-effect free.

Valid

Section titled Valid
let a;
a = 1;
Section titled Related links