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Actually, TypeScript is perfectly able to type check the code you posted. Here's what the compiler thinks:

    const functionName = disable ? "disable" : "enable"; 
    // inferred type: "disable" | "enable";
    this.form.get("foo")[functionName];
    // inferred type:
    //    FormControl["disable" | "enable"]()
    // = (FormControl["disable"] | FormControl["enable"])()
    // = (() => void)() | (() => void)()
    // = void

[Try it in the playground](https://www.typescriptlang.org/play?#code/PTAEGcHsFsFMIBYEtrgpUB3eATSA7AcgBdQEBDAN3mIxQAdIAnU8-AcwFcAbcpgKADGvcGgBizaAGECxJpG6gA3v1ChY+cgCNusABQBKZQF9VoHEnDbdhk-1NCR4ycrPtYxPZrgAuCHKQOIxU1NSYPTiZ8UHxYTFAJJmlZeW5DAG4zUwdhclFQAE1ISJloRlj8UhDQADMXAF4YuITJDP4zCysdWFLyjWJwPU7rWD8tSAVYNmCzNUECcFIaznxBYiQCADlyOFBG4e7QAH5QACID3VPQP1ONEdP00FnQEFBAmtgmcJxQYgBPeijM4XWBXAA+Zzu3Qez2IyHAADo6kkEe5PKc6pBTgYANrLVbrLY7WAAXTaoReYHen2+vwBo2er1CiWSlVSOPOlnuoAht000LJBkZYEaehZMjZCg5INOJJ5LSSErkUr59xJBkMwr2oD0tnqAD5QJRIEgcBqjBDdUYDUaTWbNRTXo1jaasvYgA)!

If you introduce a typo, you'll get nice error message:

    const functionName = disable ? "disabe" : "enable"; 
    this.form.get("foo")[functionName]();
    // error: Property 'disabe' does not exist on type 'FormControl'. Did you mean 'disable'?

Likewise if you mess up the call signature:

    this.form.get("foo")[functionName](3);
    // error: Expected 0 arguments, but got 1

So I'd say that your code is fine as is. If you are concerned about it's readability, you could move the conditional into a loop:

    for (const fieldName of ['foo', 'bar', 'baz']) {
      const control = this.form.get(fieldName);
      if (disable) {
        control.disable();
      } else {
        control.enable();
      }
    }

but whether this is an improvement is debatable.

Also note that if you just want to disable / enable all controls in a `FormGroup` or `Form`, you can simply disable / enable that `FormGroup` or `Form`, and it will propagate the signal to its children.