Enabling Strict Null Checks.md

Enabling --strictNullChecks

Start off by running the default set of strictNullCheck fixes. This will add !s as necessary to any locations that use nullable types unsafely.

Preferring ! Assertions

For example, this configuration will add !s only to *.test.ts test files, such as with VS Code's strict tests push:

{
	"fixes": {
		"strictNonNullAssertions": true
	},
	"include": ["./src/**/*.test.ts"],
	"types": {
		"strictNullChecks": true
	}
}

Preferring | null and | undefined Unions

Alternately, you can enable the incompleteTypes fix, which will prefer adding | null and | undefined to types as properties, parameters, and so on. This configuration will add those with:

• filters to ignore class and test disposal methods, as they might assign null or undefined unnecessarily
• types.matching to only add null and undefined as types
• types.onlyPrimitives to skip computing advanced types as a performance improvement

{
	"filters": [
		"CallExpression[expression.text=suiteTeardown]",
		"CallExpression[expression.text=teardown]",
		"MethodDeclaration[name.text=dispose]"
	],
	"fixes": {
		"incompleteTypes": true
	},
	"types": {
		"matching": ["^(null|undefined)$"],
		"onlyPrimitives": true,
		"strictNullChecks": true
	}
}

Many more places will need ! assertions after parameters and properties are given the new nullable types. Consider running the earlier ! fixes on your test files first, then running this on your source files.

To clean up code after this set is applied, consider using the no-non-null-assertion ESLint rule and its auto-fixer over time.