which-ts.md

Item 2: Know Which TypeScript Options You're Using

Things to Remember

• The TypeScript compiler includes several settings that affect core aspects of the language.
• Configure TypeScript using tsconfig.json rather than command-line options.
• Turn on noImplicitAny unless you are transitioning a JavaScript project to TypeScript.
• Use strictNullChecks to prevent "undefined is not an object”-style runtime errors.
• Aim to enable strict to get the most thorough checking that TypeScript can offer.

Code Samples

function add(a, b) {
  return a + b;
}
add(10, null);

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function add(a, b) {
  return a + b;
}

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function add(a, b) {
  //         ~    Parameter 'a' implicitly has an 'any' type
  //            ~ Parameter 'b' implicitly has an 'any' type
  return a + b;
}

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function add(a: number, b: number) {
  return a + b;
}

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const x: number = null;  // OK, null is a valid number

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const x: number = null;
//    ~ Type 'null' is not assignable to type 'number'

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const x: number | null = null;

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const statusEl = document.getElementById('status');
statusEl.textContent = 'Ready';
// ~~~~~ 'statusEl' is possibly 'null'.

if (statusEl) {
  statusEl.textContent = 'Ready';  // OK, null has been excluded
}
statusEl!.textContent = 'Ready';  // OK, we've asserted that el is non-null

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const tenses = ['past', 'present', 'future'];
tenses[3].toUpperCase();

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const tenses = ['past', 'present', 'future'];
tenses[3].toUpperCase();
// ~~~~~~ Object is possibly 'undefined'.

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tenses[0].toUpperCase();
// ~~~~~~ Object is possibly 'undefined'.

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