TypeScript strictNullChecks - 对象可能是'null'

Question

After enabling strictNullChecks with Angular 4.1.1, getting several errors for null checks. I have fixed bundles of them but unable to fix the same Object is possibly 'null' for this.form.get('username').value . Likewise others, I have tried the same but unable to fix the error.

if (this.form.get('username') != null) {
    body.append('username', this.form.get('username').value);
}

Answer 1

尝试使用非空断言运算符

this.form.get('username')!.value; // notice !

Answer 2

You really don't need to cast here. You actually never have to.

The most compact way to do it:

const username = this.form.get('username');
if (username) body.append('username', username.value);

Or in exit-early style:

const username = this.form.get('username');
if (!username) return;
# back to normal flow
body.append('username', username.value); // no complaint, username cannot be falsy, then it must be { value: string }

The compiler cannot infer that this.form.get('username') hasn't change between the null check and the time you use .value on it.

With a const variable, it can.