Just as important as passing when things are right is failing when things are wrong.

Here are a few examples of the types of mistakes that literate-ts can catch:

=== Unexpected errors

This has a type error that isn't marked in the source:

[source,ts]
----
const nums = [1, 2, 3];
nums.push('four');
----

It should be:

[source,ts]
----
const nums = [1, 2, 3];
nums.push('four');
       // ~~~~~~ Argument of type '"four"' is not
       //        assignable to parameter of type 'number'.
----


=== Expected errors that don't happen

If you have an error marked but TypeScript doesn't produce it, that's also a problem that requires investigation. For example:

[source,ts]
----
const nums = [1, 2, 3];
let x;
 // ~ Variable 'x' implicitly has an 'any' type.
x = 4;
nums.push(x);
----

The solution is to either remove the error comment:

[source,ts]
----
const nums = [1, 2, 3];
let x;
x = 4;
nums.push(x);
----

or adjust the example to figure out why you aren't getting the expected error. (For more on the behavior in this example, see https://effectivetypescript.com/2020/03/09/evolving-any/[Item 41: Understand Evolving any].)

=== Failing type assertions

literate-ts uses "type is" at the start of a comment to assert a type. If the type of the symbol does not match whatever is after "type is", character for character, then it's considered a failure:

[source,ts]
----
let x = 10;  // type is number
const y = 10;  // type is number
----

This will fail since the type of `y` is inferred as `10`, not `number`:

----
Failed type assertion for const y = 10; (tested y)
----

(The symbol tested is the last on which appears on the line, or the return type of a function if the last AST node is a function invocation.)

=== Output mismatches

If you're checking the output of a code sample, it has to match!

[[log-bad]]
[source,ts]
----
for (let i = 0; i < 3; i++) {
  console.log(i);
}
----

definitely doesn't log:

[[log-bad-output]]
----
1
2
3
----

When you run this through literate-ts, you'll get an error like

----
Actual output from Node did not match expected output.
----

If you open the detailed logs, you'll see the diff:

----
Actual output from Node did not match expected output.
Expected:
1
2
3
---
Actual:
0
1
2
Actual output matched expected.
----

You can also use this to check for (expected) Node.js runtime failures:

[[toupper-js]]
[source,js]
----
let city = 'new york city';
console.log(city.toUppercase());
----

will throw an error when you run it:

[[toupper-js-output]]
----
TypeError: city.toUppercase is not a function
----

=== Bad replacements

When you use the `-r` / `--replacements` flag, literate-ts will load additional code samples from that directory. These will be used in place of identically-named code samples in the text.

To make sure that these don't get out of sync, they are compared subject to a few directives. For example, this replacement:

[source,ts]
----
// HIDE
type LatLng = [number, number];
// END
function haversine(a: LatLng, b: LatLng): number {
  // COMPRESS
  return 0;
  // END
}
----

Has to appear like this in the text:

[[good-replacement]]
[source,ts]
----
function haversine(a: LatLng, b: LatLng): number {
  // ...
}
----

If these don't match up precisely, you'll get an error. Here's an example of one that fails:

[[bad-replacement]]
[source,ts]
----
function haversine(a: LatLng, b: LatLng): number {
  // ...
}
----

You'll need to run this sample with `-r examples/asciidoc/replacements` to see the failure:

----
Inline sample does not match sample in source file
----

You can also define replacements inline in your file, presumably hidden away in a comment. This is done using the `replace-with-id` directive. Here's an example of that in action.

// verifier:replace-with-id:calculate-age-replacement
[source,ts]
----
function calculateAge(birthDate: Date): number {
  // ...
}
----

////
[[calculate-age-replacement]]
[source,ts]
----
function calculateAge(birthDate: Date): number {
  // COMPRESS
  return 0;
  // END
}
----
////

And here's an example of that failing to match. Note that the replacement and the original may be defined in either order.

// verifier:replace-with-id:calculate-age-replacement
[source,ts]
----
function calculateAge(birthDate: string): number {
  // ...
}
----