TypeScript converts boolean to string

Question

I have the following problem: within the following code, I'm trying to check, if the variable result is true or false:

    const result: boolean = await this.sqlConnector.validatePassword
        (this.userData.getUserId(), validatorContext.recognized.value);
    // Returns string
    console.log(typeof(result));

The function sqlconnector.validatePassword is also returning a boolean. The header of the function looks like that:

public validatePassword (userId: string, userInput: string): Promise <boolean>

However, the typeof(result) function is saying, that the variable is from type string. This finally leads to the fact that the following if statement always fails.

    // always false because no boolean
    if (result === true) {
        return true;
    } else {

What is the reason for this?

well why does sqlConnector.validatePassword return a string? — epascarello
– epascarello, Commented Jan 28, 2019 at 21:11
I dont know :D. I have set the return value to boolean. It looks like that: return new Promise ((res, rej) => { this.con.query(getResult, (err, result) => { if (err) { rej (false) } else { res (result[0].isSecretCorrect); } }); }) In both cases, im receiving an string. — Tobias C.
– Tobias C., Commented Jan 28, 2019 at 21:14
TypeScript types are compile-time, not run-time. So it doesn't matter that you've declared result as a boolean. — Heretic Monkey
– Heretic Monkey, Commented Jan 28, 2019 at 21:17
TypeScript doesn't convert your values, since it is just JavaScript superset. And your app actually runs JS. So, if some function returns string, you'll get a string, despite boolean annotation in TS code. — Bogdan Gishka
– Bogdan Gishka, Commented Jan 28, 2019 at 21:19
Then the only answer I can come up with, that result[0].isSecretCorrect is actually a string at the moment, when it passes to the res function. — Bogdan Gishka
– Bogdan Gishka, Commented Jan 28, 2019 at 21:35

dev-dan · Accepted Answer · 2019-01-28 23:25:56Z

Looks like you are being passed back a string value. This should be fixed within the code that is returning the response. As a temporary measure you could check the string for what it is and use it in a similar way. Should hold while response if fixed.

if (typeof result === "string" && result === "true") {
    return true;
}

you could also check the response and change it accordingly.

let result: any = await this.sqlConnector.validatePassword
(this.userData.getUserId(), validatorContext.recognized.value);


if (typeof result === 'string'){
  result = result === "true";
}

Most of all though I would recommend getting the response to be returned as type Boolean if that is what is intended allowing for simple checks like.

if (result) // as long as result is true
{
  //you will end up here
}

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