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Know How to Tell Whether a Symbol Is in the Type Space or Value Space

A symbol in TypeScript exists in one of two spaces:

• Type space
• Value space

function email(to: Person, subject: string, body: string): Response {
 //      ――――― ―――――        ―――――――          ――――                     Values
 //               ――――――            ――――――        ――――――   ――――――――   Types
}

There are many other constructs that have different meanings in the two spaces:

• this in value space is JavaScript’s this keyword (Item 69). As a type, this is the TypeScript type of this, aka “polymorphic this.” It’s helpful for implementing method chains with subclasses.

• In value space, & and

  are bitwise AND and OR. In type space they are the intersection and union operators.

• In value space, const introduces a new variable, but in type space, as const changes the inferred type of a literal or literal expression (Item 20).
• In value space, extends defines a subclass (class A extends B), but in type space it defines a subtype (interface A extends B) or a constraint on a generic type (Generic).
• In value space, “in” is used in for loops (for (key in object)), while in type space it’s used in mapped types (Item 15).
• In value space, ! is JavaScript’s logical not operator (!x), but in type space it’s a non-null type assertion (x!; see Item 9).

Correct

function email(options: {to: Person, subject: string, body: string}) {
 // ...
}
function email(
 {to, subject, body}: {to: Person, subject: string, body: string}
) {
 // ...
}

ERROR

function email({
 to: Person,
 // ~~~~~~ Binding element 'Person' implicitly has an 'any' type
 subject: string,
 // ~~~~~~ Binding element 'string' implicitly has an 'any' type
 body: string
 // ~~~~~~ Binding element 'string' implicitly has an 'any' type
}) { /* ... */ }

Things to Remember

• Know how to tell whether you’re in type space or value space while reading a TypeScript expression. Use the TypeScript playground to build an intuition for this.
• Every value has a static type, but this is only accessible in type space. Type space constructs such as type and interface are erased and are not accessible in value space.
• Some constructs, such as class or enum, introduce both a type and a value.
• typeof, this, and many other operators and keywords have different meanings in type space and value space.