Strict null checks in Typescript

As a general rule, enable strictNullChecks in your TSConfig.

By default, null and undefined are assignable to all types in Typescript:

let foo: number = 123;
foo = null; // Okay
foo = undefined; // Okay

foo.bar() // May fail

This can lead to runtime errors when trying to access properties on objects that don't exist, which Typescript will not warn you about at compile time.

When using the strictNullChecks compiler option, null and undefined are not allowable by default. You can still use a union type (string | null) to accept these values.

There is also the 'Non-null assertion operator' (!.) which assers that its operand is non-null and non-undefined, e.g. foo!.bar asserts that foo is valid and we can call bar on it. This only 'ignores' the error, as it doesn't change any of the code at runtime.

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