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.

index.ts

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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.

Note that I also added an eslint comment to disable a rule which bans using ts comments in your code. You might not have to do this if you don't have this rule enabled or don't use eslint.

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

index.ts

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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.

index.ts

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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.

index.ts

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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.

index.ts

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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.

Had we not used the // @ts-expect-error comment, we would get a type checking error, because the function expects number parameters.

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

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Disable type checking for a Line in TypeScript

Disable type checking for a Line in TypeScript #

Borislav Hadzhiev