Typescript erroneously raises an issue indicating that the condition is expected to always be

Consider the code snippet below:

class A {
    private foo : number = 3;

    public method () {
        if (this.foo === 3) {
            this.anotherMethod();

            if (this.foo === 4) {
                console.log(this.foo);
            }
        }
    }

    public anotherMethod () {
        this.foo = 4;
    }
}

In this scenario, the value of foo in the class is modified by the method anotherMethod. This violates the assumption that the variable will always remain as "3". However, Typescript continues to display the error message:

error TS2367: This condition will always return 'false'
since the types '3' and '4' have no overlap.

How can I resolve this issue?

typescript

Answer №1

The insightful comment made by Jonrsharpe should definitely be further elaborated in a formal response:

class A {

  private foo : number = 3;

  public method () {

    this.foo; // Examining this.foo at this moment to determine the inferred value displays: (property) A.foo: number

    if (this.foo === 3) {

          this.anotherMethod();

          this.foo; // Analyzing this.foo at this juncture reveals: (property) A.foo: 3
                    // TypeScript restricts the type of this.foo to 3 
                    // (the numeric literal 3) instead of a number

          // You can explicitly widen it again if you desire using the 
          // `as number` Type Assertion:
          if (this.foo as number === 4) {
              console.log(this.foo);
          }
      }
  }

  public anotherMethod () {
      this.foo = 4;
  }
}

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