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TypeScript Union Types


Union types are used when a value can be more than a single type.

Such as when a property would be string or number.


Union | (OR)

Using the | we are saying our parameter is a string or number:

Example

function printStatusCode(code: string | number) {
  console.log(`My status code is ${code}.`)
}
printStatusCode(404);
printStatusCode('404');
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Union Type Errors

Note: you need to know what your type is when union types are being used to avoid type errors:

Example

function printStatusCode(code: string | number) {
  console.log(`My status code is ${code.toUpperCase()}.`) // error: Property 'toUpperCase' does not exist ontype 'string | number'.
  Property 'toUpperCase' does not exist on type 'number'
}

In our example we are having an issue invoking toUpperCase() as its a string method and number doesn't have access to it.

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TypeScript Exercises

Test Yourself With Exercises

Exercise:

Specify that the parameter "myVar" for the function can be either string or number:

function myFunc(myVar:   ) {
  console.log(myVar)
}

Start the Exercise