unbound-method

Enforce unbound methods are called with their expected scope.

Class functions don't preserve the class scope when passed as standalone variables. If your function does not access this, you can annotate it with this: void, or consider using an arrow function instead.

If you're working with jest, you can use eslint-plugin-jest's version of this rule to lint your test files, which knows when it's ok to pass an unbound method to expect calls.

Attributes

• Included in configs
  • ✅ Recommended
  • 🔒 Strict
• Fixable
  • 🔧 Automated Fixer
  • 🛠 Suggestion Fixer
• 💭 Requires type information

Rule Details

Examples of code for this rule

❌ Incorrect

class MyClass {
  public log(): void {
    console.log(this);
  }
}

const instance = new MyClass();

// This logs the global scope (`window`/`global`), not the class instance
const myLog = instance.log;
myLog();

// This log might later be called with an incorrect scope
const { log } = instance;

// arith.double may refer to `this` internally
const arith = {
  double(x: number): number {
    return x * 2;
  },
};
const { double } = arith;

✅ Correct

class MyClass {
  public logUnbound(): void {
    console.log(this);
  }

  public logBound = () => console.log(this);
}

const instance = new MyClass();

// logBound will always be bound with the correct scope
const { logBound } = instance;
logBound();

// .bind and lambdas will also add a correct scope
const dotBindLog = instance.logBound.bind(instance);
const innerLog = () => instance.logBound();

// arith.double explicitly declares that it does not refer to `this` internally
const arith = {
  double(this: void, x: number): number {
    return x * 2;
  },
};
const { double } = arith;

Options

The rule accepts an options object with the following property:

• ignoreStatic to not check whether static methods are correctly bound

ignoreStatic

Examples of correct code for this rule with { ignoreStatic: true }:

class OtherClass {
  static log() {
    console.log(OtherClass);
  }
}

// With `ignoreStatic`, statics are assumed to not rely on a particular scope
const { log } = OtherClass;

log();

Example

{
  "@typescript-eslint/unbound-method": [
    "error",
    {
      "ignoreStatic": true
    }
  ]
}

When Not To Use It

If your code intentionally waits to bind methods after use, such as by passing a scope: this along with the method, you can disable this rule.

If you're wanting to use toBeCalled and similar matches in jest tests, you can disable this rule for your test files in favor of eslint-plugin-jest's version of this rule.