This option prohibits the use of bitwise operators such as ^ (XOR), | (OR) and others. There are two reasons why you would want to enable this option:

1. Since JavaScript doesn't have integers (only doulbe precision floating-point numbers) the bitwise operators here is very slow.
2. Bitwise operations are very rare in JavaScript programs and very often & is simply a mistyped &&.

This option requires you to always put curly braces around blocks in loops and conditionals. JavaScript allows you to omit curly braces when the block consists of only one statement, for example:

while (day)
  shuffle();

However, in some circumstances, it can lead to bugs:

while (day)
  shuffle();
  sleep(); // You would think that this is a part of your loop, but it is not.

Generally, unless you are careful, it is safer to require curly braces around all blocks.

This options prohibits the use of == and != in favor of === and !==. The former try to coerce values before comparing them which can lead to some unexpected results. The latter don't do any coercion so they are generally safer. If you would like to learn more about type coercion in JavaScript, we recommend Truth, Equality and JavaScript by Angus Croll.

Note: even if this option is off, JSHint will check for unsafe comparisons like != null unless option eqnull (see below) is turned on.

This option requires all for in loops to filter objects' items. The for in statement allows for looping through the names of all of the properties of an object including those inherited throught the prototype chain. This behavior can lead to unexpected items in your object so it is generally safer to always filter inherited properties out as shown in the example:

for (key in obj) {
  if (obj.hasOwnProperty(key)) {
    // We are sure that obj[key] belongs to the object and was not inherited.
  }
}

For more in-depth understanding of for in loops in JavaScript, read Exploring JavaScript for-in loops by Angus Croll.

This option prohibits the use of immediate function invocations without wrapping them in parentheses. Wrapping parentheses assists readers of your code in understanding that the expression is the result of a function, and not the function itself.

This option prohibits the use of a variable before it was defined. JavaScript has function scope only and, in addition to that, all variables are always moved—or hoisted— to the top of the function. This behavior can lead to some very nasty bugs and that's why it is safer to always use variable only after they have been explicitly defined.

For more in-depth understanding of scoping and hoisting in JavaScript, read JavaScript Scoping and Hoisting by Ben Cherry.

This option requires you to capitalize names of constructor functions. Capitalizing functions that are intended to be used with new operator is just a convention that helps programmers to visually distinguish constructor functions from other types of functions to help spot mistakes when using this.

Not doing so won't break your code in any browsers or environments but it will be a bit harder to figure out—by reading the code—if the function was supposed to be used with or without new. And this is important because when the function that was intended to be used with new is used without it, this will point to the global object instead of a new object.

function MyConstructor() {
  console.log(this);
}

new MyConstructor(); // -> [MyConstructor]
MyConstructor();     // -> [DOMWindow]

For more in-depth understanding on how this works, read Understanding JavaScript Function Invocation and "this" by Yehuda Katz.

This option prohibits the use of arguments.caller and arguments.callee. Both .caller and .callee make quite a few optimizations impossible so they were deprecated in future versions of JavaScript. In fact, EcmaScript 5 forbids the use of arguments.callee in strict mode.

This options warns when you have an empty block in your code. JSLint was originally warning for all empty blocks and we simply made it optional. There were no studies reporting that empty blocks in JavaScript break your code in any way.

This option prohibits the use of constructor functions for side-effects. Some people like to call constructor functions without assigning its result to any variable:

new MyConstructor();

There is no advantage in this approach over simply calling MyConstructor since the object that the operator new creates isn't used anywhere so you should generally avoid constructors like this one.

This option prohibits the use of unary increment and decrement operators. Some people think that ++ and -- reduces the quality of their coding styles and there are programming languages—such as Python—that go completely without these operators.

This option prohibits the use of unsafe . in regular expressions.

This option prohibits the use of explicitly undeclared variables. This option is very useful for spotting leaking and mistyped variables.

/*jshint undef:true */

function test() {
  var myVar = 'Hello, World';
  console.log(myvar); // Oops, typoed here. JSHint with undef will complain
}

If your variable is defined in another file, you can use /*global ... */ syntaxto tell JSHint about it. See the About page for more information.

This option requires all functions to run in EcmaScript 5's strict mode. Strict mode is a way to opt in to a restricted variant of JavaScript. Strict mode eliminates some JavaScript pitfalls that didn't cause errors by changing them to produce errors and it fixes mistakes that made it difficult for JavaScript engines to perform certain optimizations.

Note: This option enables strict mode for function scope only, it prohibits the global scoped strict mode because it might break third-party widgets on your page. If you really want to use global strict mode, see the globalstrict option.

This option makes it an error to leave a trailing whitespace in your code. Trailing whitespaces can be source of nasty bugs with multi-line strings in JavaScript:

// This otherwise perfectly valid string will error if
// there is a whitespace after \
var str = "Hello \
World";

This option suppresses warnings about missing semicolons. There is a lot of FUD (fear, uncertainty and doubt) spread about semicolon spreaded by quite a few people in the community. The common myths are that semicolons are required all the time (they are not) and that they are unreliable. JavaScript has rules about semicolons which are followed by all browsers so it is up to you to decide whether you should or should not use semicolons in your code.

For more information about semicolons in JavaScript read An Open Letter to JavaScript Leaders Regarding Semicolons by Isaac Schlueter and JavaScript Semicolon Insertion.

This option suppresses warnings about the use of assignments in cases where comparisons are expected. More often than not, code like if (a = 10) {} is a typo. However, it can be useful in cases like this one:

for (var i = 0, person; person = people[i]; i++) {}

This option suppresses warnings about the debugger statements in your code.

This option suppresses warnings about == null comparisons. Such comparisons are often useful when you want to check if a variable is null or undefined.

This option tells JSHint that your code uses ECMAScript 5 specific features such as getters and setters. Note that not all browsers implement these features.

More info:

1. ES5 compatibility table by Juriy Zaytsev
2. ECMAScript 5 support in Mozilla

This option tells JSHint that your code uses ES.next specific features such as const and let. Note that these features are not finalized yet and not all browsers implement them.

More info:

1. Draft Specification for ES.next (ECMA-262 Ed. 6)

This option suppresses warnings about the use of eval. The use of eval is discouraged because it can make your code vulnerable to various injection attacks and it makes it hard for JavaScript interpreter to do certain optimizations.

This option suppresses warnings about the use of expressions where normally you would expect to see assignments or function calls. Most of the time, such code is a typo. However, it is not forbidden by the spec and that's why this warning is optional.

This option suppresses warnings about declaring variables inside of control structures while accessing them later from the outside. Even though JavaScript has only two real scopes—global and function—such practice leads to confusion among people new to the language and hard-to-debug bugs. This is way, by default, JSHint warns about variables that are used outside of their intended scope.

function test() {
  if (true) {
    var x = 0;
  }

  x += 1; // Default: 'x' used out of scope.
          // No warning when funcscope:true
}

This option suppresses warnings about the use of global strict mode. Global strict mode can break third-party widgets so it is not recommended.

For more info about strict mode see the strict option.

This option suppresses warnings about the __iterator__ property. This property is not supported by all browsers so use it carefully.

This option suppresses warnings about missing semicolons but only when the semicolon is omited for the last statement in a one-line block:

var name = (function() { return 'Anton' }());

This is a very niche use case that is useful only when you use automatic JavaScript code generators.

This option suppresses most of the warnings about possibly unsafe line breakings in your code. It doesn't suppress warnings about comma-first coding style. To suppress those you have to use laxcomma (see below).

This option suppresses warnings about comma-first coding style:

var obj = {
    name: 'Anton'
  , handle: 'valueof'
  , role: 'SW Engineer'
};

This option suppresses warnings about functions inside of loops. Defining functions inside of loops can lead to bugs such as this one:

var nums = [];

for (var i = 0; i < 10; i++) {
  nums[i] = function (j) {
    return i + j;
  };
}

nums[0](2); // Prints 12 instead of 2

To fix the code above you need to copy the value of i:

var nums = [];

for (var i = 0; i < 10; i++) {
  (function (i) {
    nums[i] = function (j) {
      return i + j;
    }
  }(i));
}

This option suppresses warnings about multi-line strings. Multi-line strings can be dangerous in JavaScript because all hell breaks loose if you accidentally put a whitespace in between the escape character (\) and a new line.

Not that even though this option allows correct multi-line strings, it still warns about multi-line strings without escape characters or with anything in between the escape character and a whitespace.

/*jshint multistr:true */

var text = "Hello\
World"; // All good.

text = "Hello
World"; // Warning, no escape character.

text = "Hello\ 
World"; // Warning, there is a space after \ (colored pink)

This option suppresses watnings about switches with just one case. Most of the time you want to use if instead of switch if there is only one case. However, some code generators prefer to generate switch statements.

This option suppresses warnings about the __proto__ property. This property is deprecated and not supported by all browsers.

This option suppresses warnings about unescaped - in the end of regular expressions.

This option suppresses warnings about the use of script-targeted URLs—such as javascript:....

This option suppresses warnings about mixed tabs and spaces when the latter are used for alignmnent only. The technique is called SmartTabs.

This option suppresses warnings about variable shadowing i.e. declaring a variable that had been already declared somewhere in the outer scope.

This option suppresses warnings about using [] notation when it can be expressed in dot notation: person['name'] vs. person.name.

This option suppresses warnings about "weird" constructions like new function () { ... } and new Object;. Such constructions are sometimes used to produce singletons in JavaScript:

var singleton = new function() {
  var privateVar;

  this.publicMethod  = function () {}
  this.publicMethod2 = function () {}
};

This option suppresses warnings about possible strict violations when the code running in a strict mode and you use this in a non-constructor function. You should use this option—in a function scope only—when you are positive that your use of this is valid in the strict mode (for example, if you call your function using Function.call).

Note: This option can be used only inside of a function scope. JSHint will fail with an error if you will try to set this option globally.

This option defines globals exposed by modern browsers: all the way from good ol' document and navigator to the HTML5 FileReader and other new developments in the browser world.

Note: this option doesn't expose variables like alert or console. See option devel for more information.

This option defines globals exposed by CouchDB. CouchDB is a document-oriented database that can be queried and indexed in a MapReduce fashion using JavaScript.

This option defines globals that are usually used for logging poor-man's debugging: console, alert, etc. It is usually a good idea to not to ship them in production because, for example, console.log breaks in legacy versions of Internet Explorer.

This option defines globals exposed by the Dojo Toolkit.

This option defines globals exposed by the jQuery JavaScript library.

This option defines globals exposed by the MooTools JavaScript framework.

This option defines globals available when your code is running inside of Node runtime environment. Node.js is a server-side JavaScript environment that uses an asynchronous event-driven model.

This option defines non-standard but widely adopted globals such as escape and unescape.

This option defines globals exposed by the Prototype JavaScript framework.

This option defines globals available when your code is running inside of Rhino runtime environment. Rhino is an open-source implementation of JavaScript written entirely in Java.

This option defines globals available when your code is running as a script for the Windows Script Host.

This option disallows the use of dangling _ in variables. We don't know why would you need it.

This option allows only one var statement per function. Some people think that having a single var in a function, at the top of the function, helps readability. Example (taken from JSLint/JSHint source code):

x.nud = function () {
  var b, f, i, j, p, seen = {}, t;

  b = token.line !== nexttoken.line;
  if (b) {
    indent += option.indent;
      if (nexttoken.from === indent + option.indent) {
        indent += option.indent;
      }
  }

  // [...]
};

This option makes JSHint stop on the first error or warning.

This option make JSHint check your source code against Douglas Crockford's JavaScript coding style. Unfortunately, his “The Good Parts” book aside, the actual rules are not very well documented.