未使用的表达式，需要赋值或 function 调用（角度）

Question

I'm making a registration form in Angular. I want to check if I have User's username and pass that value to the object if it is not null.

loadData(data: User) {
    data.username && (this.registrationData.username = data.username.trim());  
}

I am getting this error: Unused expression, expected an assignment or function call (no-unused-expression)

Answer 1

It's telling you to not abuse && as a replacement for if . Instead, use:

if (data.username) {
  this.registrationData.username = data.username.trim();
}

Or, consider:

const trimmed = data.username.trim();
if (trimmed) {
  this.registrationData.username = trimmed;
}

Answer 2

That's a common tslint rule that requires you to not leave an expression (that is, a statement that produces a result) unused.

You can use an if statement like this:

loadData(data: User) {
    if (data.username) {
        this.registrationData.username = data.username.trim();
    }
}