What does assignment in while condition check?

Question

while(a = foo())
{
    bar();
}

Is this checking:

• If assignment is successful, run bar, or
• If a is defined and non-null, run bar, or
• Something else ?

Answer 1

On every iteration, it assigns the result of the foo() call to a, and if that result was truthy, it runs the loop and tries again until the result of the call is falsey.

For example, while assigning inside a condition is generally a code smell, it's seen often enough when trying to iterate over capture groups in a global regular expression:

const str = 'abcdefg';
const pattern = /(.)./g;
let match;
while (match = pattern.exec(str)) {
  console.log('first captured group was ' + match[1]);
}
// after the last iteration, `match` is null, so the loop stops

If the assignment is not successful, and the cause of that non-success would throw an error, then the whole script stops due to the error. (unless there's a try / catch block around it)