Disable type checking for a Line in TypeScript

Disable type checking for a Line in TypeScript #

Use the // @ts-ignore comment to disable type checking for a line in TypeScript. The comment disables type checking for the next line. If you use a linter, you might need to disable it for the line on which you use the // @ts-ignore comment.

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// eslint-disable-next-line @typescript-eslint/ban-ts-comment
// @ts-ignore
function logMessage(message) {
  console.log(message);

  return message;
}

The // @ts-ignore comment disables all type checking for the next line.

If you need to disable type checking for an entire file, you can use the // @ts-nocheck comment.

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// eslint-disable-next-line @typescript-eslint/ban-ts-comment
// @ts-nocheck

function logMessage(message) {
  console.log(message);

  return message;
}

The // @ts-nocheck comment instructs TypeScript to skip type checking for the entire file.

You might also see the @ts-expect-error comment being used to disable type checking for the next line.

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// eslint-disable-next-line @typescript-eslint/ban-ts-comment
// @ts-expect-error
console.log('hello world' / 0);

The // @ts-expect-error will suppress any type errors on the next line, but if there aren't any errors, TypeScript will inform us that using @ts-expect-error was unnecessary.

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// eslint-disable-next-line @typescript-eslint/ban-ts-comment
// @ts-expect-error
console.log('hello world' / 0);

// ⛔️ Error: Unused '@ts-expect-error' directive.ts(2578)
// eslint-disable-next-line @typescript-eslint/ban-ts-comment
// @ts-expect-error
console.log(100 * 100);

That's the main difference between ts-ignore and ts-expect-error - // @ts-ignore doesn't alert us if the next line has no errors.

An example of when you would use the // @ts-expect-error comment is when you have a function that takes a parameter of a specific type, but want to test that it works as expected when passed a parameter of a different type.

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function sum(a: number, b: number) {
  if (typeof a !== 'number' || typeof b !== 'number') {
    return 0;
  }

  return a + b;
}

// eslint-disable-next-line @typescript-eslint/ban-ts-comment
// @ts-expect-error
const result = sum('hello', 'world');

if (result === 0) {
  console.log('✅ Test passes');
}

The sum function in the example takes 2 numbers as parameters, but we want to test if it works as expected when passed parameters of a different type.

Using this approach, we can test our function and would get a TypeScript error if the // @ts-expect-error comment was unnecessary.