What could be the reason for TypeScript throwing an error despite having a condition in place?

Question

What could be the reason for TypeScript throwing an error despite having a condition in place?

Having an issue with TypeScript (TS2531) and its non-null type checking. Here's the scenario:

if (this.formGroup.get('inseeActivityCode') !== null) {
    mergedCompanyActivity.inseeActivityCode = this.formGroup.get('inseeActivityCode').value; // => TS2531: Object is possibly 'null'.
}

Despite explicitly checking for inseeActivityCodeControl not being null, TypeScript still throws TS2531 error when accessing inseeActivityCodeControl.value. This is confusing as the condition should prevent this from happening. Can anyone shed light on why TypeScript continues to raise this error and offer a solution that doesn't involve using the ! assertion operator?

Thanks in advance!

angular typescript

Answer 1

Answer №1

Static code analysis only goes so far when it comes to TypeScript. The result of the get() method is sometype | null, and that's what the static code analysis uses. To ensure the result is not null, you can assign the value to a variable and check against null.

let iac = this.formGroup.get('inseeActivityCode');
if (iac !== null) {
  mergedCompanyActivity.inseeActivityCode = iac.value; 
}

Edit

Why does the assignment work but not the function call? The compiler cannot guarantee that two consecutive calls to the same method will return the same or valid value. For example:

function foo() {
  return Date.now() % 2 === 0 ? "foo" : null;
}

if (foo() !== null) { 
  let l = foo().length;
}

let f = foo();
if (f !== null) {
  let l = f.length;
}
let l2 = f.length; // Error thrown here!

By checking for !== null, the type of f narrows down from string | null to string. This type narrowing remains within the condition's body unless the value of f is reassigned. However, outside the condition's body, there is still no guarantee that f is not null.

Answer 2

Static code analysis only goes so far when it comes to TypeScript. The result of the get() method is sometype | null, and that's what the static code analysis uses. To ensure the result is not null, you can assign the value to a variable and check against null.

let iac = this.formGroup.get('inseeActivityCode');
if (iac !== null) {
  mergedCompanyActivity.inseeActivityCode = iac.value; 
}

Edit

Why does the assignment work but not the function call? The compiler cannot guarantee that two consecutive calls to the same method will return the same or valid value. For example:

function foo() {
  return Date.now() % 2 === 0 ? "foo" : null;
}

if (foo() !== null) { 
  let l = foo().length;
}

let f = foo();
if (f !== null) {
  let l = f.length;
}
let l2 = f.length; // Error thrown here!

By checking for !== null, the type of f narrows down from string | null to string. This type narrowing remains within the condition's body unless the value of f is reassigned. However, outside the condition's body, there is still no guarantee that f is not null.

What could be the reason for TypeScript throwing an error despite having a condition in place?

Answer №1

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