An expression of type 'void' cannot be tested for truthiness

# An expression of type 'void' cannot be tested for truthiness

The error "An expression of type 'void' cannot be tested for truthiness" occurs when we forget to return a value from a function and test the result for truthiness.

To solve the error, make sure to return a value from the function or correct your conditional.

expression type void cannot be tested

Here are 2 examples of how the error occurs.

index.ts

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// 👇️ function returns type void
function sum(a: number, b: number) {
  const result = a + b;
  // 👉️ forgot to return
}

// ⛔️ Error: An expression of type 'void' cannot be tested for truthiness.ts(1345)
const result = sum(10, 10) ? 20 : 0;

// ⛔️ Error: An expression of type 'void' cannot be tested for truthiness.ts(1345)
if (sum(10, 10)) {
  console.log('success');
}

expression of type void cannot be tested for truthiness

The code for this article is available on GitHub

We forgot to return a value from the sum function, so it implicitly has a return type of void.

The void type represents the return type of a function that doesn't return a value.

When you don't return a value from a function, you implicitly return undefined and the function's return type is inferred to be void.

We can't test an expression of type void for truthiness. It will always return false, because undefined is a falsy value.

# Make sure to return a value from the function

To solve the error, make sure the function you are using in the conditional returns a value.

index.ts

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// 👇️ function now returns a value
function sum(a: number, b: number): number {
  const result = a + b;

  return result; // 👈️ explicit return
}

const result = sum(10, 10) ? 20 : 0;

if (sum(10, 10)) {
  console.log('success');
}

make sure to return value from function

The code for this article is available on GitHub

Now the sum function has a return type of number.

A helpful way to debug this is to explicitly set the function's return type as in the example above. If you forget to return a value of the specified type, you will get a helpful error message.

If you're using an implicit return with an arrow function, make sure the function returns the expected value.

index.ts

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// 👇️ implicit return with objects
const getObj = () => ({
  name: 'Bobby Hadz',
  country: 'Chile',
});

// 👇️ implicit return with primitives
const getNum = (a: number, b: number) => a + b;

The code for this article is available on GitHub

If you didn't intend to test the return value of the function for truthiness, you need to update your conditional and test for something else.

If you explicitly set the function's return type but forget to return a value from the function, you'd get the A function whose declared type is neither void nor any must return a value error.

# Additional Resources

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An expression of type 'void' cannot be tested for truthiness