String validation in Typescript

Question

Coming from Javascript I need some advice on how to do string validation in Typescript.

Usually in Javascript you can just check the string as if it was a boolean, but with strong types, you get a compiler error.

A couple of solutions I thought of, but don't really like is you could do !!myString or change the return type. Is checking for null, undefined and empty string the way to do it?

See example:

function stringIsValid(myString: String) : Boolean {
    return myString; // compiler error
}

var isValid = stringIsValid(null);

Playground

@ritaj In my case it's how Javascript would validate !!myString, so not null, undefined or empty. I validate strings like if(myString){...} everywhere, so I want to know how it's usually done in Typescript. Another example is in a switch case you can't just return the string if the return type is boolean. — Mick
– Mick, Commented Jan 24, 2019 at 9:41
No, you can't "just return the string if the return type is boolean". In JavaScript, a string is not a boolean. A truthy value acts like true but is not of type boolean. developer.mozilla.org/en-US/docs/Glossary/Truthy — Paleo
– Paleo, Commented Jan 24, 2019 at 9:57

Roberto Zvjerković · Accepted Answer · 2019-01-24 13:26:56Z

3

You probably want to just use one of these, functionally they're the same:

function stringIsValid(myString: String): Boolean {
    return Boolean(myString);
}

or

function stringIsValid(myString: String): Boolean {
    return !!myString;
}

edited Jan 24, 2019 at 13:26

answered Jan 24, 2019 at 9:54

Roberto Zvjerković

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6 Comments

Mick Over a year ago

I consider !!myString bad practise in Javascript, but I like the Boolean(myString) idea. Is it considered good practise?

Paleo Over a year ago

Absolutely not. It is a bad practice to use a wrapper instead of a primitive value. An example from the Mozilla's documentation: var x = new Boolean(false); if (x) { /* this code IS executed */ }.

Roberto Zvjerković Over a year ago

@Paleo Read more carefully. Boolean(value) returns a primitive, it's not wrapped. new Boolean(value) returns a Boolean wrapper object,

Paleo Over a year ago

And the double NOT operator is a well-established practice in JavaScript and in TypeScript. developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

Paleo Over a year ago

@ritaj Ah yes my bad. The bad practice is with the constructor. Then you have to change the return type. Your function doesn't return a Boolean but a boolean.

|

Milkncookiez · Accepted Answer · 2019-01-24 15:14:44Z

3

The types from TS will not help you do runtime type validation on your variables, because TS only works at compile-time. There is a handy typeof command in JS to do type validation:

typeof myString === 'string'

The function you wrote

function stringIsValid(myString: string) : boolean {
    return myString;
}

will give you TS error at transpilation (compile-time), but these types will have no effect when you actually run your program. Here's an example of how to write it with proper typing and proper runtime check:

function stringIsValid(myString: string) : boolean {
    return typeof myString === 'string';
}

edited Jan 24, 2019 at 15:14

answered Jan 24, 2019 at 9:43

Milkncookiez

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3 Comments

Mick Over a year ago

Using typeof is verbose in comparison to Javascript and will not return false if it's an empty string. Thanks for your explanation however :)

Milkncookiez Over a year ago

You can add more checks in order to cover all possible cases for checking that the string is "valid". I guess my point was that TypeScript will not help you with that. :)

Paleo Over a year ago

Use primitive types (string, boolean) instead of wrapper types (String, Boolean) : typescriptlang.org/docs/handbook/declaration-files/…

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