TypeScript Error: Variable is Used Before Being Assigned

When working with TypeScript, you may encounter an error message that says "Variable 'x' is used before being assigned. (TS2454)". This error occurs when you use a variable before giving it a value.

1: Using a Variable Before Assigning It

Here's an example that will trigger the TS2454 error:

interface Member {
  fullName: string
  subscriptionID: number
}

let johnny: Member

if (johnny.fullName.includes('same')) {
  //🚫 ERROR TS2454:Variable 'johnny' is used before being assigned
  console.log('Fullname includes same')
}

Above, we declare a variable johnny but do not give it a value. We then use the variable in an if statement. However, since johnny has not been assigned a value yet, TypeScript throws a TS2454 error.

Solution: Initialize Variables with Default Values

To avoid the TS2454 error, you can initialize your variables with default values when you declare them. Here's an updated version of the first example, with default values:

interface Member {
  fullName: string
  subscriptionID: number
}

const johnny: Member = {
  fullName: 'Johnny',
  subscriptionID: 45631,
}

if (johnny.fullName.includes('same')) {
  console.log('Fullname includes same')
}

Here, we gave default values upon declaration. This allows us to use it in the if statement without triggering the TS2454 error.

2: Conditionally assign Variable

Another scenario that can trigger this error is when you use a variable after an if statement that assigns it a value.

Example

interface Member {
  fullName: string
  subscriptionID: number
}

let johnny: Member

if (false) {
  johnny = {
    fullName: 'Johnny',
    subscriptionID: 45631,
  }
}
console.log(johnny) //🚫 ERROR TS2454: Variable 'johnny' is used before being assigned.

Above, we declare a variable called johnny but do not give it a value. We then assign a value to johnny within an if statement Conditionally. However, since the if statement may not execute, TypeScript does not guarantee that johnny will be assigned a value. Therefore, TypeScript throws a TS2454 error when we try to use message in the console.log() statement.

Solution: Check if undefined and use default Value

Here, we check if johnny it's been defined before accessing it. If variable is defined, the expression ? johnny is evaluated, and if it's not defined, an empty object is used instead.

interface Member {
  fullName: string
  subscriptionID: number
}

let johnny: Member | undefined

if (false) {
  johnny = {
    fullName: 'Johnny',
    subscriptionID: 45631,
  }
}
console.log(johnny ? johnny : {})

Here, we use a union type of Member | undefined to tell TypeScript that johnny may be undefined. This allows TypeScript to infer the correct type for the conditional expression and avoid the TS2454 error.

Conclusion

The TS2454 error can occur when you use a variable before giving it a value. To avoid this error, you can initialize your variables with default values or check if undefined to instruct TypeScript that the variable will be assigned a value.

By following these practices, you can write safer and more reliable TypeScript code.