tslint

What is tslint?

The tslint npm package is a static analysis tool that checks TypeScript code for readability, maintainability, and functionality errors. It is widely used to enforce a consistent code style by checking the code against a set of linting rules.

What are tslint's main functionalities?

Linting TypeScript Files

This feature allows you to lint TypeScript files by specifying a configuration file and a pattern to match files. The command will process all TypeScript files in the 'src' directory and its subdirectories.

tslint -c tslint.json 'src/**/*.ts'

Fixing Linting Errors Automatically

This feature automatically fixes linting errors that can be corrected without human intervention. It is useful for fixing simple issues like whitespace or semicolon usage.

tslint --fix -c tslint.json 'src/**/*.ts'

Custom Rules

This feature allows you to use custom linting rules in addition to the predefined rules. You can specify a directory containing custom rule definitions to be applied to your code.

tslint -c tslint.json 'src/**/*.ts' --rules-dir custom_rules

Other packages similar to tslint

eslint

ESLint is a popular linting tool for JavaScript and TypeScript. It is highly configurable and extendable, with a large ecosystem of plugins. ESLint has effectively replaced TSLint as the preferred linter for TypeScript after TSLint's deprecation.

prettier

Prettier is an opinionated code formatter that supports many languages, including TypeScript. While it does not perform static code analysis, it formats code to a consistent style. Prettier can be used alongside linters like ESLint.

stylelint

Stylelint is a modern linter that helps you avoid errors and enforce conventions in your stylesheets. Although it is primarily used for CSS, it can be used in conjunction with PostCSS to lint SCSS, Sass, Less, and other CSS-like languages.

README

TSLint

A linter for the TypeScript language.

Supported Rules

A sample configuration file with all options is available here.

• ban bans the use of specific functions. Options are ["object", "function"] pairs that ban the use of object.function()
• class-name enforces PascalCased class and interface names.
• comment-format enforces rules for single-line comments. Rule options:
  • "check-space" enforces the rule that all single-line comments must begin with a space, as in // comment
    • note that comments starting with /// are also allowed, for things such as ///
  • "check-lowercase" enforces the rule that the first non-whitespace character of a comment must be lowercase, if applicable
• curly enforces braces for if/for/do/while statements.
• eofline enforces the file to end with a newline.
• forin enforces a for ... in statement to be filtered with an if statement.*
• indent enforces consistent indentation levels (currently disabled).
• interface-name enforces the rule that interface names must begin with a capital 'I'
• jsdoc-format enforces basic format rules for jsdoc comments -- comments starting with /**
  • each line contains an asterisk and asterisks must be aligned
  • each asterisk must be followed by either a space or a newline (except for the first and the last)
  • the only characters before the asterisk on each line must be whitepace characters
• label-position enforces labels only on sensible statements.
• label-undefined checks that labels are defined before usage.
• max-line-length sets the maximum length of a line.
• no-any diallows usages of any as a type decoration.
• no-arg disallows access to arguments.callee.
• no-bitwise disallows bitwise operators.
• no-console disallows access to the specified functions on console. Rule options are functions to ban on the console variable.
• no-consecutive-blank-lines disallows having more than one blank line in a row in a file
• no-construct disallows access to the constructors of String, Number, and Boolean.
• no-debugger disallows debugger statements.
• no-duplicate-key disallows duplicate keys in object literals.
• no-duplicate-variable disallows duplicate variable declarations.
• no-empty disallows empty blocks.
• no-eval disallows eval function invocations.
• no-string-literal disallows object access via string literals.
• no-trailing-comma disallows trailing comma within object literals.
• no-trailing-whitespace disallows trailing whitespace at the end of a line.
• no-unreachable disallows unreachable code after break, catch, throw, and return statements.
• no-unused-expression disallows unused expression statements, that is, expression statements that are not assignments or function invocations (and thus no-ops).
• no-unused-variable disallows unused imports, variables, functions and private class members.
  • "check-parameters" disallows unused function and constructor parameters.
    • NOTE: this option is experimental and does not work with classes that use abstract method declarations, among other things. Use at your own risk.
• no-use-before-declare disallows usage of variables before their declaration.
• no-var-requires disallows the use of require statements except in import statements, banning the use of forms such as var module = require("module")
• one-line enforces the specified tokens to be on the same line as the expression preceding it. Rule options:
  • "check-catch" checks that catch is on the same line as the closing brace for try
  • "check-else" checks that else is on the same line as the closing brace for if
  • "check-open-brace" checks that an open brace falls on the same line as its preceding expression.
  • "check-whitespace" checks preceding whitespace for the specified tokens.
• quotemark enforces consistent single or double quoted string literals.
• radix enforces the radix parameter of parseInt
• semicolon enforces semicolons at the end of every statement.
• triple-equals enforces === and !== in favor of == and !=.
• typedef enforces type definitions to exist. Rule options:
  • "callSignature" checks return type of functions
  • "catchClause" checks type in exception catch blocks
  • "indexSignature" checks index type specifier of indexers
  • "parameter" checks type specifier of parameters
  • "propertySignature" checks return types of interface properties
  • "variableDeclarator" checks variable declarations
  • "memberVariableDeclarator" checks member variable declarations
• typedef-whitespace enforces spacing whitespace for type definitions. Each rule option requires a value of "space" or "nospace" to require a space or no space before the type specifier's colon. Rule options:
  • "callSignature" checks return type of functions
  • "catchClause" checks type in exception catch blocks
  • "indexSignature" checks index type specifier of indexers
• use-strict enforces ECMAScript 5's strict mode
  • check-module checks that all top-level modules are using strict mode
  • check-function checks that all top-level functions are using strict mode
• variable-name allows only camelCased or UPPER_CASED variable names. Rule options:
  • "allow-leading-underscore" allows underscores at the beginnning.
• whitespace enforces spacing whitespace. Rule options:
  • "check-branch" checks branching statements (if/else/for/while) are followed by whitespace
  • "check-decl"checks that variable declarations have whitespace around the equals token
  • "check-operator" checks for whitespace around operator tokens
  • "check-separator" checks for whitespace after separator tokens (,/;)
  • "check-type" checks for whitespace before a variable type specification

TSLint Rule Flags

You can enable/disable TSLint or a subset of rules within a file with the following comment rule flags:

• /* tslint:disable */ - Disable all rules for the rest of the file
• /* tslint:enable */ - Enable all rules for the rest of the file
• /* tslint:disable:rule1 rule2 rule3... */ - Disable the listed rules for the rest of the file
• /* tslint:enable:rule1 rule2 rule3... */ - Enable the listed rules for the rest of the file

Rules flags enable or disable rules as they are parsed. A rule is enabled or disabled until a later directive commands otherwise. Disabling an already disabled rule or enabling an already enabled rule has no effect.

For example, imagine the directive /* tslint:disable */ on the first line of a file, /* tslint:enable:ban class-name */ on the 10th line and /* tslint:enable */ on the 20th. No rules will be checked between the 1st and 10th lines, only the ban and class-name rules will be checked between the 10th and 20th, and all rules will be checked for the remainder of the file.

Custom Rules

TSLint ships with a set of core rules that can be configured. However, users are also allowed to write their own rules, which allows them to enforce specific behavior not covered by the core of TSLint. TSLint's internal rules are itself written to be pluggable, so adding a new rule is as simple as creating a new rule file named by convention. New rules can be written in either TypeScript or Javascript; if written in TypeScript, the code must be compiled to Javascript before invoking TSLint.

Rule names are always camel-cased and must contain the suffix Rule. Let us take the example of how to write a new rule to forbid all import statements (you know, for science). Let us name the rule file noImportsRule.ts. Rules can be referenced in tslint.json in their dasherized forms, so "no-imports": true would turn on the rule.

Now, let us first write the rule in TypeScript. At the top, we reference TSLint's definition file. The exported class name must always be named Rule and extend from Lint.Rules.AbstractRule.

/// <reference path='tslint.d.ts' />

export class Rule extends Lint.Rules.AbstractRule {
	public static FAILURE_STRING = "import statement forbidden";

    public apply(syntaxTree: TypeScript.SyntaxTree): Lint.RuleFailure[] {
        return this.applyWithWalker(new NoImportsWalker(syntaxTree, this.getOptions()));
    }
}

The walker takes care of all the work.

class NoImportsWalker extends Lint.RuleWalker {
	public visitImportDeclaration(node: TypeScript.ImportDeclarationSyntax) {
		// get the current position and skip over any leading whitespace
		var position = this.position() + node.leadingTriviaWidth();

		// create a failure at the current position
		this.addFailure(this.createFailure(position, node.width(), Rule.FAILURE_STRING));

		// call the base version of this visitor to actually parse this node
		super.visitImportDeclaration(node);
	}
}

Given a walker, TypeScript's parser visits the AST using the visitor pattern. So the rule walkers only need to override the appropriate visitor methods to enforce its checks. For reference, the base walker can be found in syntaxWalker.generated.ts within the TypeScript source code.

We still need to hook up this new rule to TSLint. First make sure to compile noImportsRule.ts: tsc -m commonjs noImportsRule.ts tslint.d.ts. Then, if using the CLI, provide the directory that contains this rule as an option to --rules-dir. If using TSLint as a library or via grunt-tslint, the options hash must contain "rulesDirectory": "...". If you run the linter, you'll see that we have now successfully banned all import statements via TSLint!

Now, let us rewrite the same rule in Javascript.

function Rule() {
    Lint.Rules.AbstractRule.apply(this, arguments);
}

Rule.prototype = Object.create(Lint.Rules.AbstractRule.prototype);
Rule.prototype.apply = function(syntaxTree) {
    return this.applyWithWalker(new NoImportsWalker(syntaxTree, this.getOptions()));
};

function NoImportsWalker() {
    Lint.RuleWalker.apply(this, arguments);
}

NoImportsWalker.prototype = Object.create(Lint.RuleWalker.prototype);
NoImportsWalker.prototype.visitImportDeclaration = function (node) {
    // get the current position and skip over any leading whitespace
    var position = this.position() + node.leadingTriviaWidth();

    // create a failure at the current position
    this.addFailure(this.createFailure(position, node.width(), "import statement forbidden"));

	// call the base version of this visitor to actually parse this node
    Lint.RuleWalker.prototype.visitImportDeclaration.call(this, node);
};

exports.Rule = Rule;

As you can see, it's a pretty straightforward translation from the equivalent TypeScript code.

Finally, core rules cannot be overwritten with a custom implementation, and rules can also take in options (retrieved via this.getOptions()).

Custom Formatters

Just like rules, additional formatters can also be supplied to TSLint via --formatters-dir on the CLI or formattersDirectory option on the library or grunt-tslint. Writing a new formatter is simpler than writing a new rule, as shown in the JSON formatter's code.

/// <reference path='tslint.d.ts' />

export class Formatter extends Lint.Formatters.AbstractFormatter {
    public format(failures: Lint.RuleFailure[]): string {
        var failuresJSON: any[] = [];

        for (var i = 0; i < failures.length; ++i) {
            failuresJSON.push(failures[i].toJson());
        }

        return JSON.stringify(failuresJSON);
    }
}

Such custom formatters can also be written in Javascript. Additionally, formatter files are always named with the suffix Formatter, and referenced from TSLint without its suffix.

Installation

CLI

sudo npm install tslint -g

Library

npm install tslint

Usage

Please first ensure that the TypeScript source files compile correctly.

CLI

usage: tslint

Options:
  -c, --config  		 configuration file
  -f, --file    		 file to lint                 [required]
  -o, --out     		 output file
  -r, --rules-dir   	 rules directory
  -s, --formatters-dir   formatters directory
  -t, --format  		 output format (prose, json)  [default: "prose"]

By default, configuration is loaded from tslint.json, if it exists in the current path, or the user's home directory, in that order.

tslint accepts the following commandline options:

-f, --file:
    The location of the TypeScript file that you wish to lint. This
    option is required.

-c, --config:
    The location of the configuration file that tslint will use to
    determine which rules are activated and what options to provide
    to the rules. If no option is specified, the config file named
    tslint.json is used, so long as it exists in the path.
    The format of the file is { rules: { /* rules list */ } },
    where /* rules list */ is a key: value comma-seperated list of
    rulename: rule-options pairs. Rule-options can be either a
    boolean true/false value denoting whether the rule is used or not,
    or a list [boolean, ...] where the boolean provides the same role
    as in the non-list case, and the rest of the list are options passed
    to the rule that will determine what it checks for (such as number
    of characters for the max-line-length rule, or what functions to ban
    for the ban rule).

-o, --out:
    A filename to output the results to. By default, tslint outputs to
    stdout, which is usually the console where you're running it from.

-r, --rules-dir:
    An additional rules directory, for user-created rules.
    tslint will always check its default rules directory, in
    node_modules/tslint/build/rules, before checking the user-provided
    rules directory, so rules in the user-provided rules directory
    with the same name as the base rules will not be loaded.

-s, --formatters-dir:
    An additional formatters directory, for user-created formatters.
    Formatters are files that will format the tslint output, before
    writing it to stdout or the file passed in --out. The default
    directory, node_modules/tslint/build/formatters, will always be
    checked first, so user-created formatters with the same names
    as the base formatters will not be loaded.

-t, --format:
    The formatter to use to format the results of the linter before
    outputting it to stdout or the file passed in --out. The core
    formatters are prose (human readable) and json (machine readable),
    and prose is the default if this option is not used. Additional
    formatters can be added and used if the --formatters-dir option
    is set.

--help:
    Prints this help message.

Library

var options = {
	formatter: "json",
	configuration: configuration,
	rulesDirectory: "customRules/",
	formattersDirectory: "customFormatters/"
};

var Linter = require("tslint");

var ll = new Linter(fileName, contents, options);
var result = ll.lint();

Development

To develop tslint simply clone the repository, install dependencies and run grunt:

git clone [e-mail]:palantir/tslint.git
npm install
grunt

Creating a new Release

1. Bump up the version number in package.json and tslint.ts
2. Add a section for the new release in CHANGELOG.md
3. Run grunt to build the latest sources
4. Commit
5. Run npm publish
6. Create a git tag for the new release and push it