What causes a boolean to evaluate to true?

Question

What causes a boolean to evaluate to true?

Check out this code:

export class Smth {
  private flag: boolean;

  public update() {
    this.flag = true;

    this.inner();

    if (this.flag === false) { // Operator '===' cannot be applied to types 'true' and 'false'.
      console.log(123);
    }
  }

  private inner() {
    this.flag = false;
  }
}

I'm having trouble understanding the error in the line:

if (this.flag === false)

The TypeScript compiler is giving me an error message that says:

Operator '===' cannot be applied to types 'true' and 'false'.

But I am working with type boolean and value false.

I am currently using TypeScript version 2.6.2, but even on the online playground, I get the same result with version 2.7.

This issue is not the same as Operator '==' cannot be applied to types x and y in Typescript 2, as that problem deals with comparing constants. In my code, it involves a changeable class field and there is a function that modifies its value. Additionally, the function is being called.

this.flag = true;

this.inner(); // executes this.flag = false;

if (this.flag === false) { // ... types 'true' and 'false'. - WHY?

typescript

Answer 1

Answer №1

TypeScript showcases its intelligence by conducting static analysis on your method. It is able to identify that within this closure or context, you only ever assign the value of false. This leads to the assumption in the type definition that your variable's type is specifically false, rather than just a general boolean. TypeScript does not monitor changes within inner called methods.

Imagine the type definition as if it were declared globally in this manner:

export type boolean = true | false

where false stands alone without the presence of true.

There are various solutions to address this:

Specify the type right from the start in the class declaration, such as:
```
class MyClass {
  private flag: boolean = true
  ...
}
```
Avoid testing for direct equality and simply use the boolean itself:
```
if (!this.flag) // instead of this.flag === false
```

Answer 2

TypeScript showcases its intelligence by conducting static analysis on your method. It is able to identify that within this closure or context, you only ever assign the value of false. This leads to the assumption in the type definition that your variable's type is specifically false, rather than just a general boolean. TypeScript does not monitor changes within inner called methods.

Imagine the type definition as if it were declared globally in this manner:

export type boolean = true | false

where false stands alone without the presence of true.

There are various solutions to address this:

Specify the type right from the start in the class declaration, such as:
```
class MyClass {
  private flag: boolean = true
  ...
}
```
Avoid testing for direct equality and simply use the boolean itself:
```
if (!this.flag) // instead of this.flag === false
```

Answer 3

Answer №2

One of the challenges you are facing is a part of a larger conversation regarding the limitations of flow analysis. More information on this issue can be found here, and a similar problem has been discussed here. Essentially, the core problem revolves around:

The central question at play here is: What should we assume about the side effects of a function when it is called? There are two main schools of thought - one being more pessimistic, assuming that any function could potentially alter any object it interacts with, and the other more optimistic, assuming no state modifications from functions. However, both approaches have their drawbacks. This dilemma pertains not only to local variables but also extends to object properties.

A simple workaround for this issue is to cast constants to a general type as needed. While some may view the error messages as valuable warnings about unreachable code segments, they can also be disabled if necessary:

export class Smth {
  private flag: boolean;

  public update() {
    this.flag = true as boolean;

    this.inner();

    if (this.flag === false) { 
      console.log(123);
    }
  }

  private inner() {
    this.flag = false;
  }
}

Answer 4

One of the challenges you are facing is a part of a larger conversation regarding the limitations of flow analysis. More information on this issue can be found here, and a similar problem has been discussed here. Essentially, the core problem revolves around:

The central question at play here is: What should we assume about the side effects of a function when it is called? There are two main schools of thought - one being more pessimistic, assuming that any function could potentially alter any object it interacts with, and the other more optimistic, assuming no state modifications from functions. However, both approaches have their drawbacks. This dilemma pertains not only to local variables but also extends to object properties.

A simple workaround for this issue is to cast constants to a general type as needed. While some may view the error messages as valuable warnings about unreachable code segments, they can also be disabled if necessary:

export class Smth {
  private flag: boolean;

  public update() {
    this.flag = true as boolean;

    this.inner();

    if (this.flag === false) { 
      console.log(123);
    }
  }

  private inner() {
    this.flag = false;
  }
}

What causes a boolean to evaluate to true?

Answer №1

Answer №2

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