When it comes to data types in Typescript, an unexpected behavior may occur where an

Consider this TypeScript (2.7.2) interface:

export interface Sample {
  text?: string;
  isActive?: boolean;
}

The following code snippet will output 'false' as expected:

let sample: Sample = {text: ''};
if (sample.text && sample.text.length > 0) {
  sample.isActive = true;
} else {
  sample.isActive = false;
}
console.log(sample.isActive);

However, this piece of code prints an empty string without any value:

let sample: Sample = {text: ''};
sample.isActive = sample.text && sample.text.length > 0;
console.log(sample.isActive);

If you are puzzled by the difference in behavior between the two examples, you're not alone. The second example should theoretically work like the first one, but it doesn't. What could be the reason for this inconsistency?

Answer №1

When a.b exists and has a length greater than zero, set a.c to true because an empty string evaluates as false. To achieve this, modify the expression <code>"" && false
to evaluate "a.c".

A more organized way of coding this would be:

a.c = a.b && a.b.length > 0 ? true : false

Answer №2

This issue revolves around the concept of truthy and falsy values in Javascript, particularly related to the && operator. You can learn more about this in the official documentation.

The && and || operators do not strictly return true or false, but rather the value of one of the specified operands. This means that when used with non-Boolean values, they may yield a non-Boolean result.

Specifically, regarding the && operator:

If the first operand can be converted to false, it returns the second operand; otherwise, it returns the first operand. Therefore, when both operands are Boolean, && returns true only if both are true.

In the context of your code snippet, the empty string "" is considered falsy and can be evaluated as false. To ensure you receive a boolean output, use double negation !!:

let a: A = {b: ''};
a.c = !!(a.b && a.b.length > 0);
console.log(a.c);

Answer №3

Building on the advice from @nrgwsth, you can enhance your code structure by substituting a.b with !!a.b in your conditional statements like so:

let a: A = {b: ''};
a.c = !!a.b && a.b.length > 0;
console.log(a.c);

By using !a.b, you are essentially converting a.b to its boolean opposite. However, applying !!a.b will provide you with the positive boolean outcome of the assessment.

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