An outer value of this is shadowed by this contained in TS

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Borislav Hadzhiev

Fri Mar 25 20222 min read

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An outer value of this is shadowed by this contained in TS #

The error "An outer value of 'this' is shadowed by this container" occurs when we use named functions instead of arrow functions and shadow the value of this. To solve the error, use an arrow function, which uses this of the enclosing scope (the class instance).

outer value is shadowed by this container

Here are 2 examples of how the error occurs.

index.ts
class Employee { constructor(public salary: number) { this.salary = salary; } increaseSalary = function () { // 'this' implicitly has type 'any' because it does not have a type annotation.ts(2683) // ⛔️ index.ts(6, 20): An outer value of 'this' is shadowed by this container. this.salary += 100; return this.salary; }; // ------------------------------------------------ incSal() { // 👇️ using this in nested function function nested() { console.log(this); // 👉️ undefined // ⛔️ index.ts(14, 14): An outer value of 'this' is shadowed by this container. this.salary += 100; return this.salary; } nested(); } } const emp = new Employee(100);

We used a function expression to declare the increaseSalary method and TypeScript is not happy about our use of the this keyword inside of the function expression.

The second error is much more common - we have a nested function inside of a method. The nested function uses the this keyword, however it has a value of undefined and shadows the this keyword inside of the incSal method, which actually refers to the class instance.

To get around this, convert your named functions to arrow function instead.

index.ts
class Employee { constructor(public salary: number) { this.salary = salary; } increaseSalary = () => { // 👇️ `this` refers to class instance here this.salary += 100; return this.salary; }; // ------------------------------------------------ incSal = () => { // 👇️ this refers to class instance here const nested = () => { // 👇️ this refers to class instance here this.salary += 100; console.log(this.salary); return this.salary; }; nested(); return this.salary; }; } const emp = new Employee(100); console.log(emp.increaseSalary()); // 👉️ 200 console.log(emp.incSal()); // 👉️ 300

We used arrow functions instead of named functions when declaring our class methods.

Arrow functions use the this of the enclosing scope. In other words, the this keyword in a method that was declared using an arrow function refers to the class instance.

This is the modern and less confusing way to deal with the this keyword in classes.

The error "An outer value of 'this' is shadowed by this container" basically means that the outer value of the this keyword is shadowed (overridden in the scope).

There are other ways to get around it, e.g. by using the bind method, however arrow functions enable us to not think about the implementation detail of how the this keyword works.

Using arrow functions for class methods also make our code more readable and direct.

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