class-methods-use-this
Enforce that class methods utilize this
If a class method does not use this, it can sometimes be made into a static function. If you do convert the method into a static function, instances of the class that call that particular method have to be converted to a static call as well (MyClass.callStaticMethod())
It’s possible to have a class method which doesn’t use this, such as:
class A {
constructor() {
this.a = "hi";
}
print() {
console.log(this.a);
}
sayHi() {
console.log("hi");
}
}
let a = new A();
a.sayHi(); // => "hi"
In the example above, the sayHi method doesn’t use this, so we can make it a static method:
class A {
constructor() {
this.a = "hi";
}
print() {
console.log(this.a);
}
static sayHi() {
console.log("hi");
}
}
A.sayHi(); // => "hi"
Also note in the above examples that if you switch a method to a static method, instances of the class that call the static method (let a = new A(); a.sayHi();) have to be updated to being a static call (A.sayHi();) instead of having the instance of the class call the method.
Rule Details
This rule is aimed to flag class methods that do not use this.
Examples of incorrect code for this rule:
Open in Playground
/*eslint class-methods-use-this: "error"*/
class A {
() {
console.log("Hello World"); /*error Expected 'this' to be used by class method 'foo'.*/
}
}
Examples of correct code for this rule:
Open in Playground
/*eslint class-methods-use-this: "error"*/
class A {
foo() {
this.bar = "Hello World"; // OK, this is used
}
}
class B {
constructor() {
// OK. constructor is exempt
}
}
class C {
static foo() {
// OK. static methods aren't expected to use this.
}
static {
// OK. static blocks are exempt.
}
}
Options
This rule has two options:
"exceptMethods"allows specified method names to be ignored with this rule."enforceForClassFields"enforces that functions used as instance field initializers utilizethis. (default:true)
exceptMethods
"class-methods-use-this": [<enabled>, { "exceptMethods": [<...exceptions>] }]
The "exceptMethods" option allows you to pass an array of method names for which you would like to ignore warnings. For example, you might have a spec from an external library that requires you to overwrite a method as a regular function (and not as a static method) and does not use this inside the function body. In this case, you can add that method to ignore in the warnings.
Examples of incorrect code for this rule when used without "exceptMethods":
Open in Playground
/*eslint class-methods-use-this: "error"*/
class A {
() {
}
}
Examples of correct code for this rule when used with exceptMethods:
Open in Playground
/*eslint class-methods-use-this: ["error", { "exceptMethods": ["foo", "#bar"] }] */
class A {
foo() {
}
#bar() {
}
}
enforceForClassFields
"class-methods-use-this": [<enabled>, { "enforceForClassFields": true | false }]
The enforceForClassFields option enforces that arrow functions and function expressions used as instance field initializers utilize this. (default: true)
Examples of incorrect code for this rule with the { "enforceForClassFields": true } option (default):
Open in Playground
/*eslint class-methods-use-this: ["error", { "enforceForClassFields": true }] */
class A {
() => {}
}
Examples of correct code for this rule with the { "enforceForClassFields": true } option (default):
Open in Playground
/*eslint class-methods-use-this: ["error", { "enforceForClassFields": true }] */
class A {
foo = () => {this;}
}
Examples of correct code for this rule with the { "enforceForClassFields": false } option:
Open in Playground
/*eslint class-methods-use-this: ["error", { "enforceForClassFields": false }] */
class A {
foo = () => {}
}
Version
This rule was introduced in ESLint v3.4.0.
Further Reading