Basic Usage

TypeScript Twoslash is a pre-processor that enhances your code-samples. It is a markup language for JavaScript and TypeScript.

It leverages the compiler APIs used by text editors to offer type-driven hover information, precise error messages, and type callouts.

Using the homepage as an example, here is a simple code sample:

Before
After

```ts twoslash
// @errors: 2540

console.log((1 + 2 + 3 + 4).toFixed(2))
//                            ^|

/** A Basic Todo interface*/
interface Todo {
  title: string;
}

const todo: Readonly<Todo> = {
  title: 'Delete inactive users',
//  ^?
}

todo.title = 'Hello'
```


var console: Console
The console module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.
The module exports two specific components:

A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
A global console instance configured to write to process.stdout and
process.stderr. The global console can be used without importing the node:console module.

Warning: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the note on process I/O for
more information.
Example using the global console:
console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
Example using the Console class:
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
@see ― source
console.
Console.log(message?: any, ...optionalParams: any[]): void
Prints to stdout with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3)
(the arguments are all passed to util.format()).
const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
See util.format() for more information.
@since ― v0.1.100
log((1 + 2 + 3 + 4).to 
Number.toFixed(fractionDigits?: number): string
Returns a string representing a number in fixed-point notation.
@param ― fractionDigits Number of digits after the decimal point. Must be in the range 0 - 20, inclusive.
Fixed(2))toExponential
toFixed
toLocaleString
toPrecision
toString

/** A Basic Todo interface*/
interface 
interface Todo
A Basic Todo interface
Todo {
  
Todo.title: string
title: string;
}

const 
const todo: Readonly<Todo>
todo: 
type Readonly<T> = { readonly [P in keyof T]: T[P]; }
Make all properties in T readonly
Readonly<
interface Todo
A Basic Todo interface
Todo> = {
  title: 'Delete inactive users',
title: string
}

const todo: Readonly<Todo>
todo.title = 'Hello'Error ts(2540)  ― Cannot assign to 'title' because it is a read-only property.