'module' is not defined. Open
module.exports = config;
- Read upRead up
- Exclude checks
Disallow Undeclared Variables (no-undef)
This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var
keyword in a for
loop initializer).
Rule Details
Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/
comment.
Examples of incorrect code for this rule:
/*eslint no-undef: "error"*/
var a = someFunction();
b = 10;
Examples of correct code for this rule with global
declaration:
/*global someFunction b:true*/
/*eslint no-undef: "error"*/
var a = someFunction();
b = 10;
The b:true
syntax in /*global */
indicates that assignment to b
is correct.
Examples of incorrect code for this rule with global
declaration:
/*global b*/
/*eslint no-undef: "error"*/
b = 10;
By default, variables declared in /*global */
are read-only, therefore assignment is incorrect.
Options
-
typeof
set to true will warn for variables used inside typeof check (Default false).
typeof
Examples of correct code for the default { "typeof": false }
option:
/*eslint no-undef: "error"*/
if (typeof UndefinedIdentifier === "undefined") {
// do something ...
}
You can use this option if you want to prevent typeof
check on a variable which has not been declared.
Examples of incorrect code for the { "typeof": true }
option:
/*eslint no-undef: ["error", { "typeof": true }] */
if(typeof a === "string"){}
Examples of correct code for the { "typeof": true }
option with global
declaration:
/*global a*/
/*eslint no-undef: ["error", { "typeof": true }] */
if(typeof a === "string"){}
Environments
For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.
browser
Examples of correct code for this rule with browser
environment:
/*eslint no-undef: "error"*/
/*eslint-env browser*/
setTimeout(function() {
alert("Hello");
});
node
Examples of correct code for this rule with node
environment:
/*eslint no-undef: "error"*/
/*eslint-env node*/
var fs = require("fs");
module.exports = function() {
console.log(fs);
};
When Not To Use It
If explicit declaration of global variables is not to your taste.
Compatibility
This rule provides compatibility with treatment of global variables in JSHint and JSLint.
Further Reading
'use strict' is unnecessary inside of modules. Open
'use strict';
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- Exclude checks
Strict Mode Directives (strict)
A strict mode directive at the beginning of a script or function body enables strict mode semantics.
When used globally, the entire script, including all contained functions, are strict mode code:
"use strict";
It is also possible to specify function-level strict mode, such that strict mode applies only to the function in which the directive occurs:
function foo() {
"use strict";
return;
}
var bar = function() {
"use strict";
return;
};
Unlike scripts, ECMAScript modules are always in strict mode. Strict mode directives in ECMAScript modules have no effect.
Rule Details
This rule is aimed at using strict mode directives effectively, and as such, will flag any unexpected uses or omissions of strict mode directives.
Options
There are four options for this rule:
-
"safe"
- require"use strict"
globally when inside a module wrapper and in function scopes everywhere else. This is the default. -
"never"
- disallow"use strict"
. -
"global"
- require"use strict"
in the global scope. -
"function"
- require"use strict"
in function scopes only.
All strict mode directives are flagged as unnecessary if ECMAScript modules or implied strict mode are enabled (see [Specifying Parser Options](../user-guide/configuring#specifying-parser-options)). This behaviour does not depend on the rule options, but can be silenced by disabling this rule.
safe
Node.js and the CommonJS module system wrap modules inside a hidden function wrapper that defines each module's scope. The wrapper makes it safe to concatenate strict mode modules while maintaining their original strict mode directives. When the node
or commonjs
environments are enabled or globalReturn
is enabled in ecmaFeatures
, ESLint considers code to be inside the module wrapper, and "safe"
mode corresponds to "global"
mode and enforces global strict mode directives. Everywhere else, "safe"
mode corresponds to "function"
mode and enforces strict mode directives inside top-level functions.
never
This mode forbids any occurrence of a strict mode directive.
Examples of incorrect code for the "never"
option:
/*eslint strict: ["error", "never"]*/
"use strict";
function foo() {
"use strict";
return;
}
var bar = function() {
"use strict";
return;
};
foo();
bar();
Examples of correct code for the "never"
option:
/*eslint strict: ["error", "never"]*/
function foo() {
return;
}
var bar = function() {
return;
};
foo();
bar();
global
This mode ensures that all code is in strict mode and that there are no extraneous strict mode directives at the top level or in nested functions, which are themselves already strict by virtue of being contained in strict global code. It requires that global code contains exactly one strict mode directive. Strict mode directives inside functions are considered unnecessary. Multiple strict mode directives at any level also trigger warnings.
Examples of incorrect code for the "global"
option:
/*eslint strict: ["error", "global"]*/
"use strict";
"use strict";
function foo() {
"use strict";
return function() {
"use strict";
"use strict";
return;
};
}
foo();
Examples of correct code for the "global"
option:
/*eslint strict: ["error", "global"]*/
"use strict";
function foo() {
return function() {
return;
};
}
foo();
function
This mode ensures that all function bodies are strict mode code, while global code is not. Particularly if a build step concatenates multiple scripts, a strict mode directive in global code of one script could unintentionally enable strict mode in another script that was not intended to be strict code. It forbids any occurrence of a strict mode directive in global code. It requires exactly one strict mode directive in each function declaration or expression whose parent is global code. Strict mode directives inside nested functions are considered unnecessary. Multiple strict mode directives at any level also trigger warnings.
Examples of incorrect code for the "function"
option:
/*eslint strict: ["error", "function"]*/
"use strict";
function foo() {
// Missing strict mode directive
return function() {
"use strict"; // Unnecessary; parent should contain a strict mode directive
"use strict";
return;
};
}
foo();
Examples of correct code for the "function"
option:
/*eslint strict: ["error", "function"]*/
function foo() {
"use strict";
return function() {
return;
};
}
(function() {
"use strict";
return;
}());
foo();
earlier default (removed)
Replacement notice: This mode, previously enabled by turning on the rule without specifying a mode, has been removed in ESLint v1.0. "function"
mode is most similar to the deprecated behavior.
This mode ensures that all functions are executed in strict mode. A strict mode directive must be present in global code or in every top-level function declaration or expression. It does not concern itself with unnecessary strict mode directives in nested functions that are already strict, nor with multiple strict mode directives at the same level.
Examples of incorrect code for an earlier default option which has been removed:
// "strict": "error"
function foo() {
return true;
}
Examples of correct code for an earlier default option which has been removed:
// "strict": "error"
"use strict";
function foo() {
return true;
}
// "strict": "error"
function foo() {
"use strict";
return true;
}
// "strict": "error"
(function() {
"use strict";
// other code
}());
When Not To Use It
In a codebase that has both strict and non-strict code, either turn this rule off, or selectively disable it where necessary. For example, functions referencing arguments.callee
are invalid in strict mode. A full list of strict mode differences is available on MDN.
Source: http://eslint.org/docs/rules/