Why does TypeScript not complain in strict mode about the following scenarios?
function test(firstName: string, lastName?: string): string {
return firstName + " " + lastName;
}
test('John');Or this?
const str: string = '';
const num: object = {};
const result: string = str + num;I find it strange that TypeScript doesn't raise errors for cases like 'John undefined' or '[object Object]'. Shouldn't type checking be able to catch such issues? (Unlike Flow)
According to MartinJohns, in a discussion on a GitHub issue related to this topic
This issue may be better suited for linters rather than being classified as a type error. It seems to be more about undesirable behavior.
Develop a robust variadic function that ensures type safety while performing string concatenation.
concat(...strings: string[]): string {
var concatenated = "";
for (var i = 0; i < strings.length; i++) {
concatenated += strings[i];
}
return concatenated;
};
Utilize your function when you need to concatenate strings:
const str: string = '';
const num: object = {};
const result: string = concat(str, num); // triggers an error
When using the + operator, it is not possible to restrict it to specific types because it is designed to support mixed types, as seen in expressions like "Count: " + a.length().
In modern JavaScript, object-to-string conversion can be inferred and controlled. An example of this can be:
let o = {};
o.toString = ()=> "two"
alert("one" + " " + o); // Displays "one two"Therefore, rejecting an example simply based on types might not be necessary.
Regarding conversion from unknown and null to string, this operation is implied and defined in the abstract ToString() function, which is well-defined in the ECMAScript standard. While tools like tslint may raise flags, it remains a valid operation in ECMAScript.
One interesting aspect is the ability to combine strings and undefined values. The same case applies to your second illustration. In JavaScript, you are able to concatenate '' + {} (although this may not be very practical).
Enforcing strict typing helps avoid risky operations such as accessing lastName.length in the scenario you described.
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