I am used to using something like the following pattern on a dynamic language to check/assert type:
function square(num) {
// pretend we don't already have a TypeError
if (!num instanceof Number) throw new MyCustomError("Only a number is allowed here")
return num * num;
}
However, js doesn't allow instanceof on primitives, well, at least in a way that helps check the type easily such as I'd want in the above. What is the recommended/idiomatic way to do this, just compare to a string? Such as:
// create our own error
class ValidationError extends Error {
constructor(message) {
super(message);
this.name = "ValidationError";
}
}
function square(num) {
// pretend we don't want a TypeError
if (typeof num != 'number')
throw new ValidationError("Only a number is allowed here")
return num * num;
}
console.log(square(2));
console.log(square('abc'));Create a reusable decorator function that makes any function which only accepts numeric parameters throw an error if called with non-numeric arguments:
function multiply(a, b) { return a * b }
function numArgsOnly(fn) {
return function(...args) {
if (args.some(n => n!==Number(n)))
throw `${args.find(n => n!==Number(n))} is not a number`;
return fn(...args);
}
}
const safeMultiply = numArgsOnly(multiply);
console.log(safeMultiply(3,9));
try {
safeMultiply(3,'foo');
} catch (e) { console.error(e) }Instead of checking if a variable is not a Number by doing an instanceof check (which does not work on primitives), try the following.
const num = 78;
console.log(isNaN(num) || typeof num !== "number");In here, we are using the typeof keyword, as it is supported by JavaScript primitives. We are also checking if the value is NaN.