# no-cond-assign

Forbid the use of assignment expressions in conditions which may yield unwanted behavior.

Assignment expressions return the value assigned:

let foo = 5;
console.log(foo = 8); // 8
console.log(foo += 4) // foo + 4 (12 in this case)

Users often make a typo and end up using = instead of == or === in conditions in statements like if, while, do_while, and for. This is erroneous and is most likely unwanted behavior since the condition used will actually be the value assigned.

# Incorrect Code Examples

let foo = 5;
if (foo = 6) {
//      ^^^ assignments return the value assigned, therefore the condition checks `6`
//          `6` is always truthy, therefore the if statement always runs even if we dont want it to.
} else {}
//^^^^ it makes this else unreachable
foo // 6

# Config

if (foo = 54) {}

while (foo = 1) {}

do { /* */ } while (bar = 1)

for(;foo = 4; bar) {}

if (bar = 5 ? foo : bar) {}

Source (opens new window)