This decorator can be applied to properties on a class to ensure that they conform to guards when they are set. The guards also act as a mechanism for casting values from HTML attributes, which makes for nicer component APIs.
import { Value } from '@trademe/ensure';
export class MyClass {
@Value(isOne) mustBeOne: number;
@Value(isString, isLengthFive) verySpecificString: string;
}
export function isOne (value: any) {
if (value === 1) {
return value;
}
throw new Error();
}
export function isString (value: string) {
if (typeof value === 'string') {
return value;
}
throw new Error();
}
export function isLengthFive (value: string) {
if (value.length === 5) {
return value;
}
throw new Error();
}
Ensure that a value is not null
. isNotNull
is special in that it acts when getting the value, not setting it.
import { isNotNull } from '@trademe/ensure';
export class MyClass {
@Value(isNotNull) mustNotBeNull: boolean;
}
let instance = new MyClass();
instance.mustNotBeNull = true; // works
instance.mustNotBeNull = null; // works
console.log(instance.mustNotBeNull): // throws!
Ensure that a value is boolean. Also casts from 'true'
or ''
to true
and 'false'
to false
.
import { isBool } from '@trademe/ensure';
export class MyClass {
@Value(isBool) mustBeBoolean: boolean;
}
let instance = new MyClass();
instance.mustBeBoolean = true; // works
instance.mustBeBoolean = 'hello'; // throws!
instance.mustBeBoolean = 'false'; // works
console.log(instance.mustBeBoolean) // false
isBool
is particularly useful for simplifying component APIs when combined with @Input
.
import { isBool } from '@trademe/ensure';
@Component({ ... })
export class MyComponent {
@Input() @Value(isBool) mustBeBoolean: boolean;
}
<my-component
mustBeBoolean><!-- No more pesky ="true"! -->
</my-component>
Ensure that a value is a valid enum value.
import { isEnum } from '@trademe/ensure';
export enum MyEnum {
foo,
bar
}
export class MyClass {
@Value(isEnum<typeof MyEnum>(MyEnum)) mustBeEnum: MyEnum;
}
let instance = new MyClass();
instance.mustBeEnum = 'foo'; // works
console.log(instance.mustBeEnum); // 0
instance.mustBeEnum = 'bar'; // works!
console.log(instance.mustBeEnum); // 1
instance.mustBeEnum = 'baz'; // throws!
Ensure that a value is a valid number.
import { isNumber } from '@trademe/ensure';
export class MyClass {
@Value(isNumber) mustBeNumber: number;
}
let instance = new MyClass();
instance.mustBeNumber = 5; // works
console.log(instance.mustBeNumber); // 5
instance.mustBeNumber = '1.33'; // works!
console.log(instance.mustBeNumber); // 1.33
instance.mustBeNumber = 'baz'; // throws!
Sometimes it is useful to write your own guards. To do that, we provide an ensure
method, which you need to wrap your rule with:
import { ensure, EnsureError } from '@trademe/ensure';
export const isInt = ensure((value: any, key: string): number => {
const num = +parseInt(value, 10);
if (isNaN(num)) {
throw new EnsureError(`
'${key}' must be a number.
`);
}
if (`${num}` !== `${value}`) {
throw new EnsureError(`
'${key}' must be an integer.
`);
}
return num;
});
If you need to do something more complicated, say with arguments, the inner function should be wrapped with ensure
. You can also indicate that a guard should be run when the property is get
rather than set
:
import { ensure, EnsureError } from '@trademe/ensure';
export function isMutuallyExclusive (mutuallyExclusiveKey: string): Function {
return ensure((value: any, key: string): any {
let mutuallyExclusiveValue = this[mutuallyExclusiveKey];
if (mutuallyExclusiveValue != null && mutuallyExclusiveValue !== false) {
throw new EnsureError(`
"${key}" and "${mutuallyExclusiveKey}" cannot be used at the same time.
`);
}
return value;
}, {
getter: true
});
}