Hack University - Literal Types, any, and unknown

Types can be exact values

A literal type is a type with exactly one allowed value. "left" (the type) accepts only "left" (the string). Alone that is not useful, but combined with | (or) it models a fixed menu of options:

let direction: "left" | "right" = "left";
direction = "right"; // fine
direction = "up";    // error: not assignable

Compare the JavaScript habit of passing magic strings and hoping nobody misspells "rigth". The | symbol builds a union type, and Unit 4 is devoted to it. For now, remember: strings and numbers themselves can be types, and a union of literals is TypeScript's built-in answer to "one of these exact values".

Code exercise · typescript

Run it. Then try assigning direction = "up" and run again, the compiler lists the two allowed values in its error.

any and unknown: checking off vs. checking kept on

Two special types describe "could be anything", with opposite safety:

	`any`	`unknown`
Assign anything to it	yes	yes
Call methods on it	yes, unchecked	no, error
Type checking	switched off	still on

any turns the compiler off for that value: x.foo.bar() compiles and crashes at runtime, the exact bugs you came here to escape. unknown also accepts any value, but keeps checking on: every use is an error until you prove what the value is, with a check like typeof value === "string".

When does a value genuinely have an unknowable type? When it comes from outside your program. The classic source is a web API — a program on another server that yours sends a request to over the network and gets data back from, usually as JSON text. JSON.parse turns that text into a value, but nothing guarantees the other server sent the shape you expect. Type the result unknown and prove its shape before use; Unit 7 builds that full pattern.

any silences the compiler entirely. unknown accepts any value but keeps every check active until you narrow it.

Code exercise · typescript

This function takes unknown and proves the value is a string before touching .length. Run it, then try moving the return "a string..." line above the if to see the compiler refuse.

Code exercise · typescript

Your turn. Write flip(state: "on" | "off"): "on" | "off" that returns the other value. Then declare bulb starting at "off", flip it twice, and print "bulb is " + bulb after each flip.

Quiz

Your program parses text it received from a web API (another server answering your program's request) with JSON.parse, and you do not yet know the shape. Which type should the variable have?