Assert Util
Some assertion utility functions.
Installation
npm install --save @gmjs/assertUsage
All functions are exported as named exports.
import { invariant } from '@gmjs/assert';
const x: string | undefined = 'some value';
invariant(x !== undefined, 'x must be defined');API
ensureNever
Used for compile time check to make sure that the value provided to the function is of type never.
This can be used to make sure, at compile time, that all cases for a type are handled.
If you have the following:
type Operation = 'add' | 'subtract';
function compute(operation: Operation, op1: number, op2: number): number {
switch (operation) {
case 'add': {
return op1 + op2;
}
case 'subtract': {
return op1 - op2;
}
default: {
return ensureNever(operation);
}
}
}And then you decide to change Operation:
type Operation = 'add' | 'subtract' | 'multiply' | 'divide';compute function will now complain at compile time until you add cases handling the new 'multiply' and 'divide' options, and so exhaustive handling will be ensured at compile time.
After we add the new cases, everything works again:
type Operation = 'add' | 'subtract' | 'multiply' | 'divide';
function compute(operation: Operation, op1: number, op2: number): number {
switch (operation) {
case 'add': {
return op1 + op2;
}
case 'subtract': {
return op1 - op2;
}
case 'multiply': {
return op1 * op2;
}
case 'divide': {
return op1 / op2;
}
default: {
return ensureNever(operation);
}
}
}ensureNotNull
Throws an error if the given value is null, otherwise returns the given value, with the type narrowed to exclude null.
const x: string | null = 'some value';
const y = ensureNotNull(x);
// y is equal to 'some value', and has type stringconst x: string | null = null;
const y = ensureNotNull(x); // this line will throwensureNotNullish
Throws an error if the given value is null or undefined, otherwise returns the given value, with the type narrowed to exclude null and undefined.
const x: string | null | undefined = 'some value';
const y = ensureNotNullish(x);
// y is equal to 'some value', and has type stringconst x: string | null | undefined = null;
const y = ensureNotNullish(x); // this line will throwconst x: string | null | undefined = undefined;
const y = ensureNotNullish(x); // this line will throwensureNotUndefined
Throws an error if the given value is undefined, otherwise returns the given value, with the type narrowed to exclude undefined.
const x: string | undefined = 'some value';
const y = ensureNotUndefined(x);
// y is equal to 'some value', and has type stringconst x: string | undefined = undefined;
const y = ensureNotUndefined(x); // this line will throwinvariant
Throws an error if the given condition is not truthy.
It will also narrow the type of any variable used in the condition.
const x: string | undefined = 'some value';
invariant(x !== undefined, 'x must be defined');
// from here on, x is narrowed to string typeconst x: string | undefined = undefined;
invariant(x !== undefined, 'x must be defined'); // this line will throw