Type safety means:

your code is prevented from using values in ways that don’t match what they actually are.

In simpler terms, the language and compiler help stop you from doing “wrong things with data” before the code ever runs.

A concrete example

Not type-safe (plain JavaScript)

function double(x) {
  return x * 2;
}

double("hello"); // no error, returns NaN

The bug shows up at runtime, possibly in production.

Type-safe (TypeScript)

function double(x: number): number {
  return x * 2;
}

double("hello"); // compile-time error

The compiler stops you before the app even runs.

What “safe” actually means here

Type safety ensures:

• You can’t call a function with the wrong type
• You can’t access properties that don’t exist
• You must handle null / undefined
• Different states are modeled explicitly
• Refactors don’t silently break things