TypeScript Style Guide

This is the TypeScript style guide that we use internally at Platypi! It is semi-reasonable, but it's more important that we keep a consistent look/feel of our code.

Table of Contents

0. Introduction
1. Browser Compatibility
2. Files
3. Indentation
4. Line Length
5. Quotes
6. Commas
7. Comments 0. Class 0. Inline 0. Todo and XXX
8. Variable Declarations
9. Function Declarations 0. Anonymous Functions
10. Names 0. Variables, Modules, and Functions 0. Use of var, let, and const 0. Types 0. Classes 0. Interfaces 0. Constants
11. Statements 0. Simple 0. Compound 0. Return 0. If 0. For 0. While 0. Do While 0. Switch 0. Try 0. Continue 0. Throw
12. Iterators
13. Whitespace
14. Object and Array Literals
15. Destructuring
16. Assignment Expressions
17. === and !== Operators
18. Promises and Async/Await
19. Typings 0. External 0. Internal
20. Eval
21. TSLint
22. Prettier
23. Helper Files
24. License

Introduction

When developing software as an organization, the value of the software produced is directly affected by the quality of the codebase. Consider a project that is developed over many years and handled/seen by many different people. If the project uses a consistent coding convention it is easier for new developers to read, preventing a lot of time/frustration spent figuring out the structure and characteristics of the code. For that purpose, we need to make sure we adhere to the same coding conventions across all of our products. This will not only help new developers, but it will also aid in quickly identifying potential flaws in the code, thereby reducing the brittleness of the code.

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Browser Compatibility

• Target evergreen browsers ie >= 11
• Target modern browsers ie >= 9 if it is necessary for a project
• Avoid targeting older browsers ie < 9 if at all possible

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Files

• All TypeScript files must have a ".ts" or ".tsx" extension.
• They should be all lower case, and only include letters, numbers, and periods.
• It is OK (even recommended) to separate words with periods (e.g. my.view.html).
• All files should end in a new line. This is necessary for some Unix systems.

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Indentation

• The unit of indentation is four spaces.
• Never use tabs, as this can lead to trouble when opening files in different IDEs/Text editors. Most text editors have a configuration option to change tabs to spaces.

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Line Length

• Lines must not be longer than 140 characters.
• When a statement runs over 140 characters on a line, it should be broken up, ideally after a comma or operator.

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Quotes

• Use single-quotes '' for all strings, and use double-quotes "" for strings within strings.
• When you need to use an apostrophe inside a string it is recommended to use double-quotes.
• Use template literals only when using expression interplation ${}

// bad
let greeting = "Hello World!";

// good
let greeting = 'Hello World!';

// bad
let phrase = 'It\'s Friday!';

// good
let phrase = "It's Friday!";

// bad
let html = "<div class='bold'>Hello World</div>";

// bad
let html = '<div class=\'bold\'>Hello World</div>';

// good
let html = '<div class="bold">Hello World</div>';

// bad
let template = `string text string text`;

// good
let template = `string text ${expression} string text`;

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Commas

• Use trailing commas always. DO NOT USE leading commas.
• Always use an additional trailing comma

// bad
const person = {
    firstName: 'John'
  , lastname: 'Smith'
  , email: 'john.smith@outlook.com'
};

// bad
const person = {
    firstName: 'John',
    lastname: 'Smith',
    email: 'john.smith@outlook.com'
};

// good
const person = {
    firstName: 'John',
    lastname: 'Smith',
    email: 'john.smith@outlook.com',
};

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Comments

• Comments are strongly encouraged. It is very useful to be able to read comments and understand the intentions of a given block of code.
• Comments need to be clear, just like the code they are annotating.
• Make sure your comments are meaningful.

The following example is a case where a comment is completely erroneous, and can actually make the code harder to read.

// Set index to zero.
let index = 0;

• All public functions must have a comment block /**...*/ using JSDoc style comments.

JSDocs can be interpreted by IDEs for better intellisense. Below is an example of a JSDoc comment block for a function.

/**
 * Takes in a name and returns a greeting string.
 *
 * @param name The name of the greeted person.
 */
function getGreeting(name: string): string {
    return 'Hello ' + name + '!';
}

Class

• All classes must have block comments /**...*/ for all public variables and functions.
• All public functions should use JSDoc style comments.
• Functions need to have a comment explaining what the function does, and all of the input parameters need to be annotated with @param.
• The class should include a block comment containing the description of the class
• The constructor should contain a JSDoc comment annotating any input parameters.

/**
 * Contains properties of a Person.
 */
class Person {
    /**
     * Returns a new Person with the specified name.
     *
     * @param name The name of the new Person.
     */
    public static GetPerson(name: string): Person {
        return new Person(name);
    }

    /**
     * @param name The name of the new Person.
     */
    constructor(public name: string) { }

    /**
     * Instructs this Person to walk for a certain amount
     * of time.
     *
     * @param millis The number of milliseconds the Person
     * should walk.
     */
    public walkFor(millis: number): void {
        console.log(this.name + ' is now walking.');

        setTimeout(() => {
            console.log(this.name + ' has stopped walking.');
        }, millis);
    }
}

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Inline

• Inline comments are comments inside of complex statements (loops, functions, etc).
• Use // for all inline comments.
• Keep comments clear and concise.
• Place inline comments on a newline above the line they are annotating
• Put an empty line before the comment.

// bad
let lines: Array<string>; // Holds all the lines in the file.

// good
// Holds all the lines in the file.
let lines: Array<string>;

// bad
function walkFor(name: string, millis: number): void {
    console.log(name + ' is now walking.');
    // Wait for millis milliseconds to stop walking
    setTimeout(() => {
        console.log(name + ' has stopped walking.');
    }, millis);
}

// good
function walkFor(name: string, millis: number): void {
    console.log(name + ' is now walking.');

    // Wait for millis milliseconds to stop walking
    setTimeout(() => {
        console.log(name + ' has stopped walking.');
    }, millis);
}

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Todo and XXX

TODO and XXX annotations help you quickly find things that need to be fixed/implemented.

• Use // TODO: to annotate solutions that need to be implemented.
• Use // XXX: to annotate problems the need to be fixed.
• It is best to write code that doesn't need TODO and XXX annotations, but sometimes it is unavoidable.

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Variable Declarations

• All variables must be declared prior to using them. This aids in code readability and helps prevent undeclared variables from being hoisted onto the global scope.

// bad
console.log(a + b);

let a = 2;
let b = 4;

// good
let a = 2;
let b = 4;

console.log(a + b);

• Implied global variables should never be used.
• You should never define a variable on the global scope from within a smaller scope.

// bad
function add(a: number, b: number): number {
    // c is on the global scope!
    c = 6;

    return a + b + c;
}

• Declare each variable on a newline.
• Use let or const to declare each variable. This can save you a lot of trouble when refactoring.

// bad
let a = 2,
    b = 2,
    c = 4;

// good
let a = 2;
let b = 2;
let c = 4;

// bad
// b will be defined on global scope.
let a = b = 2, c = 4;

Function Declarations

• All functions must be declared before they are used.
• Any closure functions should be defined right after the let declarations.

// bad
function createGreeting(name: string): string {
    let message = 'Hello ';

    return greet;

    function greet() {
        return message + name + '!';
    }
}

// good
function createGreeting(name: string): string {
    let message = 'Hello ';

    function greet() {
        return message + name + '!';
    }

    return greet;
}

• There should be no space between the name of the function and the left parenthesis ( of its parameter list.
• There should be one space between the right parenthesis ) and the left curly { brace that begins the statement body.

// bad
function foo (){
    // ...
}

// good
function foo() {
    // ...
}

• The body of the function should be indented 4 spaces.
• The right curly brace } should be on a new line.
• The right curly brace } should be aligned with the line containing the left curly brace { that begins the function statement.

// bad
function foo(): string {
  return 'foo';}

// good
function foo(): string {
    return 'foo';
}

• For each function parameter
  • There should be no space between the parameter and the colon : indicating the type declaration.
  • There should be a space between the colon : and the type declaration.

// bad
function greet(name:string) {
  // ...
}

// good
function greet(name: string) {
  // ...
}

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Anonymous Functions

• All anonymous functions should be defined as fat-arrow/lambda () => { } functions unless it is absolutely necessary to preserve the context in the function body.
• All fat-arrow/lambda functions should have parenthesis () around the function parameters.

// bad
clickAlert() {
    let element = document.querySelector('div');

    this.foo = 'foo';

    element.addEventListener('click', function(ev: Event) {
        // this.foo does not exist!
        alert(this.foo);
    });
}

// good
clickAlert() {
    let element = document.querySelector('div');

    this.foo = 'foo';

    element.addEventListener('click', (ev: Event) => {
        // TypeScript allows this.foo to exist!
        alert(this.foo);
    });
}

• Always surround the function block with braces {}

// bad
element.addEventListener('submit', ev => ev.preventDefault());

// bad
element.addEventListener('submit', (ev: Event) => ev.preventDefault());

// good
element.addEventListener('submit', (ev: Event) => {
    ev.preventDefault();
});

• There should be a space between the right parenthesis ) and the =>
• There should be a space between the => and the left curly brace { that begins the statement body.

// bad
element.addEventListener('click', (ev: Event)=>{alert('foo');});

// good
element.addEventListener('click', (ev: Event) => {
    alert('foo');
});

• The statement body should be indented 4 spaces.
• The right curly brace } should be on a new line.
• The right curly brace } should be aligned with the line containing the left curly brace { that begins the function statement.

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Names

• All variable and function names should be formed with alphanumeric A-Z, a-z, 0-9 and underscore _ charcters.

Variables, Modules, and Functions

• Variable, module, and function names should use lowerCamelCase.

Use of var, let, and const

• Use const where appropriate, for values that never change
• Use let everywhere else
• Avoid using var

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Types

• Always favor type inference over explicit type declaration except for function return types
• Always define the return type of functions. This can help catch errors as the functions evolve.
• Types should be used whenever necessary (no implicit any).
• Arrays should be defined as type[] instead of Array<type> .
• Use the any type sparingly, it is always better to define an interface.

// bad
let numbers = [];

// bad
let numbers: Array<number> = [];

// good
let numbers: number[] = [];

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Classes

• Classes/Constructors should use UpperCamelCase (PascalCase).
• Private and private static members in classes should be denoted with the private keyword.
• Protected members in classes do not use the private keyword.
• Default to using protected for all instance members
• Use private instance members sparingly
• Use public instance members only when they are used by other parts of the application.

class Person {
    protected fullName: string;

    constructor(public firstName: string, public lastName: string) {
        this.fullName = firstName + ' ' + lastName;
    }

    public toString() {
        return this.fullName;
    }

    protected walkFor(millis: number) {
        console.log(this.fullName + ' is now walking.');

        // Wait for millis milliseconds to stop walking
        setTimeout(() => {
            console.log(this.fullName + ' has stopped walking.');
        }, millis);
    }
}

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Interfaces

• Interfaces should use UpperCamelCase.
• Interfaces should be prefaced with the capital letter I.
• Only public members should be in an interface, leave out protected and private members.

interface IPerson {
    firstName: string;
    lastName: string;
    toString(): string;
}

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Constants

• All constants you be defined with the const keyword.

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Statements

Simple

• Each line should contain at most one statement.
• A semicolon should be placed at the end of every simple statement.

// bad
let greeting = 'Hello World'

alert(greeting)

// good
let greeting = 'Hello World';

alert(greeting);

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Compound

Compound statements are statements containing lists of statements enclosed in curly braces {}.

• The enclosed statements should start on a newline.
• The enclosed statements should be indented 4 spaces.

// bad
if (condition === true) { alert('Passed!'); }

// good
if (condition === true) {
    alert('Passed!');
}

• The left curly brace { should be at the end of the line that begins the compound statement.
• The right curly brace } should begin a line and be indented to align with the line containing the left curly brace {.

// bad
if (condition === true)
{
    alert('Passed!');
}

// good
if (condition === true) {
    alert('Passed!');
}

• Braces {} must be used around all compound statements even if they are only single-line statements.

// bad
if (condition === true) alert('Passed!');

// bad
if (condition === true)
    alert('Passed!');

// good
if (condition === true) {
    alert('Passed!');
}

If you do not add braces {} around compound statements, it makes it very easy to accidentally introduce bugs.

if (condition === true)
    alert('Passed!');
    return condition;

It appears the intention of the above code is to return if condition === true, but without braces {} the return statement will be executed regardless of the condition.

• Compount statements do not need to end in a semicolon ; with the exception of a do { } while(); statement.

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Return

• If a return statement has a value you should not use parenthesis () around the value.
• The return value expression must start on the same line as the return keyword.

// bad
return
    'Hello World!';

// bad
return ('Hello World!');

// good
return 'Hello World!';

• It is recommended to take a return-first approach whenever possible.

// bad
function getHighestNumber(a: number, b: number): number {
    let out = b;

    if(a >= b) {
        out = a;
    }

    return out;
}

// good
function getHighestNumber(a: number, b: number): number {
    if(a >= b) {
        return a;
    }

    return b;
}

• Always explicitly define a return type. This can help TypeScript validate that you are always returning something that matches the correct type.

// bad
function getPerson(name: string) {
    return new Person(name);
}

// good
function getPerson(name: string): Person {
    return new Person(name);
}

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If

• Alway be explicit in your if statement conditions.

// bad
function isString(str: any) {
    return !!str;
}

// good
function isString(str: any): str is string {
    return typeof str === 'string';
}

Sometimes simply checking falsy/truthy values is fine, but the general approach is to be explicit with what you are looking for. This can prevent a lot of unncessary bugs.

If statements should take the following form:

if (/* condition */) {
    // ...
}

if (/* condition */) {
    // ...
} else {
    // ...
}

if (/* condition */) {
    // ...
} else if (/* condition */) {
    // ...
} else {
    // ...
}

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For

For statements should have the following form:

for(/* initialization */; /* condition */; /* update */) {
    // ...
}

let keys = Object.keys(/* object */);
let length = keys.length;

for(let i = 0; i < length; i += 1) {
    // ...
}

Object.prototype.keys is supported in ie >= 9.

• Use Object.prototype.keys in lieu of a for...in statement.

With TypeScript you can use for...of statements:

let arr = [2, 3, 4];

for (const value of arr) {
    // ...
}

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While

While statements should have the following form:

while (/* condition */) {
    // ...
}

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Do While

• Do while statements should be avoided unless absolutely necessary to maintain consistency.
• Do while statements must end with a semicolon ;

Do while statements should have to following form:

do {
    // ...
} while (/* condition */);

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Switch

Switch statements should have the following form:

switch (/* expression */) {
    case /* expression */:
        // ...
        /* termination */
    default:
        // ...
}

• Each switch group except default should end with break, return, or throw.

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Try

• Try statements should be avoided whenever possible. They are not a good way of providing flow control.
• Try statements should be used when using async/await syntax.

Try statements should have the following form:

try {
    // ...
} catch (error: Error) {
    // ...
}

try {
    // ...
} catch (error: Error) {
    // ...
} finally {
    // ...
}

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Continue

• It is recommended to take a continue-first approach in all loops.

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Throw

• Avoid the use of the throw statement unless absolutely necessary.
• Flow control through try/catch exception handling is not recommended.

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Whitespace

Blank lines improve code readability by allowing the developer to logically group code blocks. Blank spaces should be used in the following circumstances.

• A keyword followed by left parenthesis ( should be separated by 1 space.

// bad
if(condition) {
    // ...
}

// good
if (condition) {
    // ...
}

• All operators except for period ., left parenthesis (, and left bracket [ should be separated from their operands by a space.

// bad
let sum = a+b;

// good
let sum = a + b;

// bad
let name = person . name;

// good
let name = person.name;

// bad
let item = items [4];

// good
let item = items[4];

• No space should separate a unary/incremental operator !x, -x, +x, ~x, ++x, --x and its operand.

// bad
let neg = - a;

// good
let neg = -a;

• Each semicolon ; in the control part of a for statement should be followed with a space.

// bad
for(let i = 0;i < 10; i += 1) {
    // ...
}

// good
for(let i = 0; i < 10; i += 1) {
    // ...
}

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Iterators

Don't use iterators whenever it's possible to use higher-order functions.

// bad
const numbers = [1, 2, 3];
let sum = 0;

for (let num of numbers) {
    sum += num;
}

// good
const numbers = [1, 2, 3];
const sum = numbers.reduce((prev, current) => {
    return prev + current;
}, 0);

// bad
const number = [1, 2, 3];
const add1 = [];
for (let i = 0; i < numbers.length; i += 1) {
  add1.push(numbers[i] + 1);
}

// good
const number = [1, 2, 3];
const add1 = numbers.map((num) => {
    return num + 1;
});

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Object and Array Literals

• Use curly braces {} instead of new Object().
• Use brackets [] instead of new Array().

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Destructuring

• Use Object destructuring always.
• Use Array destructuring except when returning

// bad
function toName(user) {
    const firstName = user.firstName;
    const lastName = user.lastName;
    const email = user.email;

    if (isEmpty(lastName)) {
        if (isEmpty(firstName)) {
            return email;
        }

        return firstName;
    }

    return `${firstName} ${lastName}`;
}

// good
function toName({ firstName, lastName, email }) {
    if (isEmpty(lastName)) {
        if (isEmpty(firstName)) {
            return email;
        }

        return firstName;
    }

    return `${firstName} ${lastName}`;
}

const arr = [1, 2, 3, 4];

// bad
const first = arr[0];
const second = arr[1];

// good
const [first, second] = arr;

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Assignment Expressions

• Assignment expressions inside of the condition block of if, while, and do while statements should be avoided.

// bad
while (node = node.next) {
    // ...
}

// good
while (typeof node === 'object') {
    node = node.next;

    // ...
}

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=== and !== Operators

• It is better to use === and !== operators whenever possible.
• == and != operators do type coercion, which can lead to headaches when debugging code.

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Promises and Async/Await

• Always use async/await wherever possible. Avoid using Promise.then and Promise.catch.

// bad
function reddits(): Promise<reddit.IListing> {
    return fetch('https://www.reddit.com/r/all.json')
        .then((response) => {
            return response.json();
        })
        .then((result) => {
            return result.data;
        })
        .catch((error) => {
            console.log(error);

            return {
                kind: 'Listing',
                data: {
                    children: [],
                },
            };
        });
}

// good
async function reddits(): Promise<reddit.IListing> {
    try {
        const response = await fetch('https://www.reddit.com/r/all.json');

        return response.json();
    } catch(error) {
        console.log(error);

        return {
            kind: 'Listing',
            data: {
                children: [],
            },
        };
    }
}

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Typings

External

• Typings are sometimes packaged with node modules, in this case you don't need to do anything
• Use @types for all external library declarations not included in node_modules
• Actively add/update/contribute typings when they are missing

Internal

• Create declaration files .d.ts for your interfaces instead of putting them in your .ts files
• Let the TypeScript compiler infer as much as possible
• Avoid defining types when it is unnecessary

// bad
let a: number = 2;

// good
let a = 2;

• Always define the return type of functions, this helps to make sure that functions always return the correct type

// bad
function sum(a: number, b: number) {
    return a + b;
}

// good
function sum(a: number, b: number): number {
    return a + b;
}

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Eval

• Never use eval
• Never use the Function constructor
• Never pass strings to setTimeout or setInterval

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TSLint

• Always use a Linter

Linting your code is very helpful for preventing minor issues that can escape the eye during development. We use TSLint (written by Palantir) for our linter.

• TSLint: https://github.com/palantir/tslint
• Our tslint.json
• Use tslint-microsoft-contrib (included in our tslint.json)

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Prettier

• Always use a formatter

Formatting your code is very helpful for readability. When working in teams, it is nice to be able to look at code that is in the same format across the entire application.

• Prettier: https://github.com/prettier/prettier

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Project Helper Files

We have provided a package.json that can serve has a starter for a TypeScript project. It has the following things:

• TypeScript
  • typescript is listed as a dependency... obviously!
  • 3 tsconfig.json files are included, one for a node server, one for client files, and one to use with text editors like VSCode or Atom
  • The tsconfig.json files are configured with rules that closely match the rules defined in this guide.
• TSLint
  • tslint and tslint-microsoft-contrib are listed as devDependencies
  • a lint script that you can use in your projects with npm run lint
• Prettier
  • prettier is listed as a devDependency
  • a prettier.config.js file has some rule for prettier that closely match the rules defined in this guide.
  • a format script is included that you can modify to suit your needs. It will call on prettier to format all the TypeScript files in the src directory by default.

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License

(The MIT License)

Copyright (c) 2014 Platypi, LLC

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.