Careeroza

Non-null Assertion

The ! Operator

Putting ! after an expression removes null and undefined from its type. Use it only when you are certain the value exists.

Example

const prices = new Map<string, number>([["SKU-1", 499], ["SKU-2", 89]]);

// Map.get returns number | undefined
const a = prices.get("SKU-1")!;  // we just inserted it, so it exists
console.log(a + 1);

const b = prices.get("SKU-2");
if (b !== undefined) console.log(b + 1); // safer: check instead of !

Output

500
90

When ! Lies

Example

The compiler trusts the !, so this compiles but crashes at runtime with a TypeError.

const prices = new Map<string, number>([["SKU-1", 499]]);
const missing = prices.get("SKU-404")!;
console.log(missing.toFixed(2));

Output

Uncaught TypeError: Cannot read properties of undefined (reading 'toFixed')